MORPHOLOGY


THE OVUM

Sphingid ova are generally green or yellow in colour, smooth and shiny with no obvious surface sculpturing, although under high magnification a slight reticulate pattern may be evident. They are either spherical or oval, slightly flattened dorso-ventrally.

The micropyle, through which the male sperm fertilizes the egg, is lateral. The egg is not always proportionate to the size of the moth which lays it, that of Agrius convolvuli, for example, being about the same size as that of Macroglossum stellatarum, i.e. about 1mm in diameter. The largest eggs are those of species in which the adult does not feed, such as Marumba quercus and Laothoe populi. These vary in diameter from 2--3mm. Freshly laid, most are some shade of green or yellow, although the waterproof sticky substance which attaches them to the hostplant or exposure to sunlight may discolour them. Very few are patterned, an exception being those of Rethera komarovi, which are green with an encircling white band (Pittaway, 1979a). The normally thick, translucent egg-shell or chorion changes to some shade of grey or yellow prior to hatching, such coloration being due to the colours of the developing larva. When the larva is about to emerge it becomes very active, twisting and turning within the egg-shell; the dark mandibles of green larvae can be seen biting at the interior at this stage.


THE LARVA

The larval stage is primarily concerned with the efficient acquisition of food to promote growth. As the outer cuticle has a limited ability to stretch, the larva can grow only by moulting, which it does three, four or five times. Each moult is called an ecdysis and the intervals between these are known as instars (often expressed as L1, L2, L3, etc.).

The body is cylindrical, or anteriorly tapering and consists of thirteen segments in addition to a capsular head which itself consists of six fused sclerotized segments. Caterpillars are polypodous larvae, that is, their abdomen bears a series of false legs (prolegs) in addition to the three pairs of jointed thoracic limbs, the true legs. The former terminate in a single semicircular crown of hooks (crotchets) and are situated on abdominal segments 3--6 and on segment 10. The latter terminate in sharp claws and are found on thoracic segments 1, 2 and 3. The head is either oval, rounded or triangular, although most first instar-larvae have spherical heads. The highest region of the head is called the vertex; behind the vertex is the occiput, with a small triangular sinus situated dorsally on the hinder margin and called the occipital sinus; the front of the head is called the face, the side the cheek, and the underside of the head behind the mouth-parts the gula or throat (Bell & Scott, 1937). A triangular clypeus occupies the centre of the face. The mouthparts and immediate surrounding area contain most of the gustatory and sensory organs. Six stemmata (often incorrectly referred to as ocelli) are situated at the base of each cheek and gula, just above the base of the antennae. They are simple, round and convex, and are capable of limited vision. The antennae, one at the base of each cheek, are composed of three segments, all of which can be retracted telescopically. These are used by the caterpillar to feel its way and for taste. A pair of strong, truncated, hollow mandibles are set in sockets near the base of the antennae. These are used not only for eating but also, in some species, for self-defence. Directly in front of the mandibles lies a transverse, grooved plate composed of the labrum and ligula, which is used to guide plant material between the mandibles, for tasting and to stop food particles falling out from between the mandibles. The maxillolabial complex below and behind the mandibles is responsible not only for the main sense of taste but also bears pairs of maxillary and labial palpi, and the all important spinneret.

The eighth abdominal segment bears a caudal horn, but in the final instar of some species this may be absent or replaced by a tubercle or 'button'. There is a spiracle situated laterally on either side of thoracic segment 1 and abdominal segments 1--8. The cuticle bears sparse, fine, secondary hairs and, in the Sphinginae, the larvae may also have raised transverse ridges edged with small tubercles in some or all instars. Larvae of all the western Palaearctic Sphingidae are generally similar in shape although they may exhibit an amazing variety of patterns and colours, not only between species but also intraspecifically. In a significant proportion of species, the larva rests with the anterior segments raised clear of the substrate and the head tucked underneath. This feature gives the group its alternative common name of 'Sphinx moths'.


THE PUPA

The pupa or chrysalis also consists of the head and thirteen segments, although many of these are indistinct and the divisions arbitrary. The head is anterior to the thorax, which comprises the prothorax (thoracic segment 1), the mesothorax (segment 2), and the metathorax (segment 3); the abdomen comprises abdominal segments 1--10. The pupal body is usually smooth, short and fusiform, with the head often distinctly narrower than the thorax; the fifth, sixth and seventh abdominal segments are movable.

The front of the head is called the frons, the lower part the clypeus. This is not the same structure as in the larva, although the name is used for a similar region. The base of the proboscis case adjoins the clypeus, and this structure can be cariniform (keel-shaped), enlarged basally, or a partially free, jughandle-like lobe. However, in most species it lies flush with the surrounding segments and extends to the tip of, and narrowly separates, the two wings. The top of the head adjoining segment 2 is termed the vertex; this separates the two large eyes and gives rise to a pair of antennae which run down either side between the wings and the mid-legs. The mesothorax merges laterally with the forewings, these covering the ventral surface of the first four abdominal segments (1--4). The abdomen terminates in a prominent triangular cremaster. A spiracle is found either side on the prothorax, and there is a spiracle on each side of abdominal segments 2--8.

Sphingid pupae have characteristic differences. Cuticle colours range from a pale cream in Macroglossum stellatarum, through various browns to a deep black in Laothoe populi. The pupa may be very glossy (e.g. Rethera komarovi), have a shiny, rugose finish (e.g. Marumba quercus), or be matt black or brown (e.g. Mimas tiliae).


THE ADULT

The adult moth or imago, the final stage of the lepidopterous life cycle, has an easily distinguishable head, thorax and abdomen.

With very few exceptions, species of the Sphingidae can be distinguished from other Lepidoptera by their general appearance. They are medium- to large-sized moths with relatively long and slender wings, a stout thorax and a long conical abdomen: in bulk, these moths are amongst the largest in the world.


THE HEAD

The head bears a pair of large compound eyes, above each of which arises a long, three-segmented antenna composed of the first segment (scape), the second segment (pedicel) and the third segment (flagellum), which often terminates in a small reflexed tip. The antennae are well provided with sensory receptors (sensilla) along the unscaled ventral surface. In the male, the ventral surface usually also has pronounced fasciculate setae, but in the female these setae are simple.

The compound eyes are separated posteriorly by the occiput, dorsally by the vertex (which may bear ocelli) and anteriorly by the frons. At the base of the frons (clypeus), either side of the proboscis base, lie a pair of pilifers and atrophied maxillary palps. The coiled and tubular proboscis (haustellum) is derived from modified maxillary galeae. When coiled, this is protected on either side by an enlarged, heavily scaled, three-segmented labial palp. In those species with a functional proboscis, the tip is provided with chemoreceptors (papillae).

In comparison with other moths, one internal feature of sphingid anatomy is remarkable -- namely the head musculature (Fleming, 1968). Most sphingids have powerful cranial muscles, the origins of which appear to be independent of ectodermal structures. The musculature varies between species but as many as eighteen muscles may be found inside the head in addition to the intrinsic proboscis and antennal muscles. Most are paired and can be divided into six groups, the details of which are beyond the scope of this work (see Eaton, 1988).


THE THORAX

The thorax is composed of three more or less fused segments, each bearing a pair of legs. The prothorax (which bears a pair of membranous patagia which protect the 'neck') is greatly reduced and lies in front of the highly developed mesothorax which, in addition to the paired forewings, supports two tegulae covering the wing bases. Lastly, the smaller metathorax bears the smaller hindwings. Each thoracic segment is composed of a series of rigid plates joined together by flexible membranes; dorsally the plates are known as tergites, ventrally as sternites and laterally as pleurites. The whole thorax is densely covered with insulating hair-like scales.

Typically, the three pairs of legs consist of five sections: the coxa, trochanter, femur, tibia and tarsus. The foretibia on the inner surface bears a movable lamellate spur or lobe (epiphysis) which is used to clean the eyes and antennae. The tarsi are usually five-segmented with a pretarsus, or terminal segment, of paired apical claws. In most species, a pad-like pulvillus is present between the claws which is richly endowed with chemoreceptors. Each claw is also flanked on its outer side by a paronychium. Large spurs are frequently present on the mid- and hindtibiae.


THE WINGS

The wings of the Sphingidae, are heavily scaled and generally triangular in shape: the forewing is long and narrow; the hindwing is small. As in all Lepidoptera, the wings consist of a double membrane supported by a network of tubular veins. At both family and generic level the wing venation has common features. For taxonomic and descriptive purposes, wings are divided into clearly defined areas and the veins designated by letter and number according to one or other system of nomenclature. That of Comstock & Needham (1898), with minor modifications, has been adopted in this work. Among Macrolepidoptera, all Sphingidae have the following unique combination of features: vein R1 of the hindwing crosses to vein Sc from the middle of the cell, the base of vein M1 is missing, and vein 1A (regarded as the posterior cubital vein CuP in some modern works) is absent. The forewing lacks the first anal vein 1A. Following Rothschild & Jordan (1903) and Bell & Scott (1937), the three parts of the angled distal cross-vein bordering the discoidal cell separating M1 and M2, M2 and M3, and M3 and Cu1, are referred to as D2, D3 and D4, respectively. The frenulum (a brush of bristles in the female, a single strong bristle in the male), which links the hindwing to the forewing, is usually well developed, although atrophied in a number of the Smerinthini.


THE ABDOMEN

The abdomen is cylindrical, fusiform and consists of ten segments (as in the larva); in the male, the last two are fused together and modified to form the external genitalia. Each segment bears a dorsal plate, the tergum, and a ventral plate, the sternum, joined laterally by a membranous pleural area bearing the spiracles.


THE GENITALIA

The genital organs are of diagnostic importance in the taxonomic study of the Sphingidae as they differ both between genera and species. In the male, there is a roughly triangular dorsal tegumen derived from the ninth tergum (T9). It abuts the tenth tergum (T10), which is a single or double hooked process (the uncus). Ventrally, the ninth sternum (S9) has been transformed into an arch-like vinculum, which may extend anteriorly as a median process (or saccus). Laterally, the ninth sternum bears plate-like claspers (valvae), which vary in form. The tenth sternum (S10) is usually jaw-shaped -- hence the name 'gnathos' -- and with the uncus it forms a sclerotized ring around the anal tube. The penis (aedeagus) lies in the centre of this structure between the valvae.

In the female, the genitalia are much simpler. The anus and oviposition pore are flanked by a pair of sensitive, hairy pads (papillae anales) situated on the ninth segment. The copulatory opening (ostium bursae) lies between the seventh and eighth segments and leads into the abdomen along a duct (ductus bursae) to the corpus bursae. It is here that the spermatophore is deposited during mating.


MORPHOLOGICAL VARIATION

Interspecific variation is used for taxonomic purposes and this is discussed in the chapter on classification. There are also, however, small differences between individuals of the same species to the extent that no two hawkmoths are exactly alike. Two main factors produce individual variation:



Unlike genetic variability, that produced by environmental factors cannot be passed on to the next generation, although selection pressures can alter the genetic bias of a population by favouring certain genotypes. Several populations may demonstrate a range of individual variability and still interbreed to give viable offspring; these constitute a species (or subspecies).

A species in which three or more colour forms, or morphs, share the same range is termed polymorphic. For example, adult Mimas tiliae are polymorphic and many of these morphs have been given names.

Sexual dimorphism is the commonest variation within a population and is due to the different complement of sex chromosomes -- in Lepidoptera, males are XX, females XY or X0 (the opposite of mammals). The main differences are usually in the colour, shape and size of the wings and antennae. For example, females of Laothoe populi are always larger than the males, lighter in colour (often rosy), and bear narrow, filiform antennae, whereas the antennae of the male have fasciculate setae ventrally.

The same genotype can produce, under differing environmental conditions, different phenotypes. Heat applied to the developing pupae of Hyles euphorbiae results in beautiful pink-flushed moths. Those of Mimas tiliae, if chilled considerably, yield the dark montane form, f. montana Daniel. Similarly, the dry parched conditions of the eastern Mediterranean produce smaller, paler individuals of Theretra alecto than is normal in India. However, when reared in more humid environments and given succulent hostplants, Greek specimens are indistinguishable from Asian material. Conversely, specimens of Hemaris tithymali deserticola reared under optimum conditions retain their unique patterning and coloration; they do not take on the appearance of Hyles tithymali mauretanica (Pittaway, 1993). In central Europe all second generation specimens of Henaris fuciformis have a toothed border in the forewings (f. milesiformis Trimen), which is almost always absent in first generation individuals.

Infrequently, individuals occur in which both male and female characters are present; these are termed gynandromorphs. Some have these characters scattered over the body in a mosaic; others are divided bilaterally, with one side male and the other wholly female. Bilateral gynandromorphs are found not infrequently in Laothoe populi.


HYBRIDS

Natural hybrids amongst certain species of wild sphingids are not uncommon, especially when these are closely related. This is hardly surprising considering that the female sex pheromones of all genera are similar in composition. Additionally, some genera have separated only comparatively recently into their present constituent species and hence still have many features in common, such as compatible sexual appendages, similar ranges, overlapping flight periods, similar hostplants. Most hybrids are to be found within Smerinthus, Laothoe, Hyles and Deilephila, with the first and last pairs of genera being sufficiently closely related to allow intergeneric, as opposed to interspecific, crosses.


Natural hybrid between Hyles euphorbiae euphorbiae and Hyles vespertilio, Austria.

However, unlike plant hybrids which exhibit hybrid vigour in being stronger and larger than their parents, hybrid hawkmoths are usually weaker and sterile, and often exhibit abnormal behaviour, e.g. pupae of Hyles x Deilephila rarely overwinter. When parent species are sufficiently closely related to give fertile offspring, the latter's fertility is greatly reduced and often ceases at the F2 generation.

Adult hybrids tend to be a mixture of their parents, both in size and markings (Pfaff, 1931; Pittaway, 1993). In most, these markings are fused together, while in some they exist separately, side by side. Full grown hybrid larvae may display a mixture of parental coloration, or be so different as to give the impression that they are of a distinct species (Pfaff, 1931).

The main requisites for hybridization to occur have been listed above. In the cross, Hyles euphorbiae euphorbiae male x Hyles vespertilio female, Europe's commonest sphingid hybrid, it has been demonstrated that the emergence of one species should be earlier than, but overlap with, that of the other species. Hyles vespertilio emerges 8--14 days before Hyles euphorbiae, thus older Hyles vespertilio males, which emerge before their own females (as do the males of Hyles euphorbiae), are presented with receptive females of their own species but not of Hyles euphorbiae. Fresh Hyles euphorbiae males have an initial choice of receptive females of Hyles vespertilio, followed by females of their own species.

Most of the hybrids shown in the table (Table 1) have been produced artificially, either by confining females of two species with males of one of them (Dannenberg, 1942), or by siphoning body fluids and cell clusters from one pupa and injecting them into another (Meyer, 1953). Some, however, occur naturally in the wild and these have been indicated by an asterisk(*).


Table 1: Table of all known natural (*) or artificially-produced sphingid hybrids. The first part of the table gives hybrids sorted by female to the right, the second part hybrids sorted by male.
MALE FEMALE
Smerinthini Smerinthini
Mimas tiliae Smerinthus o. ocellatus
Smerinthus kindermannii Laothoe p. populi
S. o. ocellatus Mimas tiliae
S. o. ocellatus S. o. atlanticus
S. o. ocellatus L. p. populi*
S. o. atlanticus S. o. ocellatus
S. o. atlanticus L. p. populi
S. o. atlanticus L. austauti
Laothoe p. populi S. kindermannii*
L. p. populi S. o. ocellatus*
L. p. populi L. austauti
L p. populi L. amurensis*
L. austauti S. o. ocellatus
L. austauti S. o. atlanticus*
L. austauti L. p. populi
L. amurensis L. p. populi*
Macroglossini Macroglossini
Hyles e. euphorbiae Hyles t. tithymali*
H. e. euphorbiae H. t. mauretanica
H. e. euphorbiae H. dahlii
H. e. euphorbiae H. n. nicaea
H. e. euphorbiae H. g. gallii*
H. e. euphorbiae H. vespertilio*
H. e. euphorbiae H. h. hippophaes*
H. e. euphorbiae Deilephila e. elpenor*
H. e. euphorbiae D. p. porcellus
H. t. tithymali H. e. euphorbiae
H. t. tithymali H. g. gallii
H. t. mauretanica H. e. euphorbiae
H. t. mauretanica H. g. gallii
H. t. mauretanica D. e. elpenor
H. t. deserticola H. e. euphorbiae
H. dahlii H. e. euphorbiae
H. dahlii D. e. elpenor
H. c. centralasiae H. t. deserticola
H. g. gallii H. e. euphorbiae*
H. g. gallii H. t. mauretanica
H. g. gallii H. t. deserticola
H. g. gallii H. dahlii
H. g. gallii H. vespertilio
H. g. gallii H. h. hippophaes
H. g. gallii H. l. livornica
H. g. gallii D. e. elpenor
H. n. nicaea H. e. euphorbiae
H. n. castissima H. t. mauretanica*
H. z. zygophylli H. l. livornica*
H. vespertilio H. e. euphorbiae*
H. vespertilio H. g. gallii
H. vespertilio H. h. hippophaes*
H. vespertilio D. e. elpenor
H. vespertilio D. p. porcellus
H. h. hippophaes H. e. euphorbiae
H. h. hippophaes H. t. tithymali
H. h. hippophaes H. t. deserticola
H. h. hippophaes H. g. gallii
H. h. hippophaes H. vespertilio
H. h. hippophaes D. e. elpenor
H. l. livornica H. e. euphorbiae
H. l. livornica H. t. tithymali
H. l. livornica H. g. gallii
H. l. livornica H. z. zygophylli*
H. l. livornica D. e. elpenor
Deilephila e. elpenor H. e. euphorbiae*
D. e. elpenor H. t. tithymali
D. e. elpenor H. t. mauretanica
D. e. elpenor H. g. gallii
D. e. elpenor H. vespertilio*
D. e. elpenor H. h. hippophaes
D. e. elpenor H. l. livornica
D. e. elpenor D. p. porcellus*
D. p. porcellus H. e. euphorbiae
D. p. porcellus D. e. elpenor
FEMALE MALE
Smerinthini Smerinthini
Mimas tiliae Smerinthus o. ocellatus
Smerinthus kindermannii Laothoe p. populi*
S. o. ocellatus Mimas tiliae
S. o. ocellatus S. o. atlanticus
S. o. ocellatus L. p. populi*
S. o. ocellatus L. austauti*
S. o. atlanticus S. o. ocellatus
S. o. atlanticus L. austauti
Laothoe p. populi S. kindermannii
L. p. populi S. o. ocellatus*
L. p. populi S. o. atlanticus
L. p. populi L. austauti
L. p. populi L. amurensis*
L. austauti S. o. atlanticus
L. austauti L. p. populi
L. amurensis L. p. populi*
Macroglossini Macroglossini
Hyles e. euphorbiae Hyles t. tithymali
H. e. euphorbiae H. t. mauretanica
H. e. euphorbiae H. t. deserticola
H. e. euphorbiae H. dahlii
H. e. euphorbiae H. g. gallii*
H. e. euphorbiae H. n. nicaea
H. e. euphorbiae H. vespertilio*
H. e. euphorbiae H. h. hippophaes
H. e. euphorbiae H. l. livornica
H. e. euphorbiae Deilephila e. elpenor*
H. e. euphorbiae D. p. porcellus
H. t. tithymali H. e. euphorbiae*
H. t. tithymali H. h. hippophaes
H. t. tithymali H. l. livornica
H. t. tithymali D. e. elpenor
H. t. mauretanica H. e. euphorbiae
H. t. mauretanica H. g. gallii
H. t. mauretanica H. n. castissima*
H. t. mauretanica D. e. elpenor
H. t. deserticola H. c. centralasiae
H. t. deserticola H. g. gallii
H. t. deserticola H. h. hippophaes
H. dahlii H. e. euphorbiae
H. dahlii H. g. gallii
H. g. gallii H. e. euphorbiae*
H. g. gallii H. t. tithymali
H. g. gallii H. t. mauretanica
H. g. gallii H. vespertilio
H. g. gallii H. h. hippophaes
H. g. gallii H. l. livornica
H. g. gallii D. e. elpenor
H. n. nicaea H. e. euphorbiae
H. z. zygophylli H. l. livornica*
H. vespertilio H. e. euphorbiae*
H. vespertilio H. g. gallii
H. vespertilio H. h. hippophaes
H. vespertilio D. e. elpenor*
H. h. hippophaes H. e. euphorbiae*
H. h. hippophaes H. g. gallii
H. h. hippophaes H. vespertilio*
H. h. hippophaes D. e. elpenor
H. l. livornica H. g. gallii
H. l. livornica H. z. zygophylli*
H. l. livornica D. e. elpenor
Deilephila e. elpenor H. e. euphorbiae*
D. e. elpenor H. t. mauretanica
D. e. elpenor H. g. gallii
D. e. elpenor H. vespertilio
D. e. elpenor H. h. hippophaes
D. e. elpenor H. l. livornica
D. e. elpenor D. p. porcellus
D. p. porcellus H. e. euphorbiae
D. p. porcellus H. vespertilio
D. p. porcellus D. e. elpenor*



SPECIES CONCEPTS

A species can be described as an aggregate of populations which share similar genetic material and hence have a similar appearance. They are also capable of interbreeding naturally to produce viable, self-perpetuating offspring. Many processes can lead to the formation of a species; all stop or restrict the flow of genetic material between portions of the range or population of a species.

Bearing in mind the amount of genetic variability within a species, if it has a small, continuous range, or is highly mobile, the results of any selection pressure exerted on part or all of that range will be rapidly transferred throughout the populations and all individuals will look similar, e.g. Macroglossum stellatarum. However, most species are not migratory and have, or may have had, a large range. Selection pressure at either end of this range, or on different parts of it, may vary, resulting in recognizably different populations -- Hyles euphorbiae is a good example of this.

Where there is a gradual change in characters (markings or colour) from one end of a continuous range to another, this is termed a cline. The populations are still genetically interconnected and resemble one another. For instance, Clarina kotschyi gradually changes from a small, reddish moth with wavy wing-margins in Lebanon to a much larger, browner subspecies with straight wing-margins in Turkey and Iran. That the two subspecies are closely related with identical early stages has already been proven (Pittaway, 1982b). Each breeds true when reared under identical environmental conditions and, when cross-paired, produce perfectly viable offspring.

Where the differences between two populations are so marked and characteristic that they are distinguishable from one another yet are still capable of interbreeding, these populations are usually termed subspecies, although there are certain circumstances when this definition can be very subjective (see Hyles euphorbiae and Hyles tithymali). Subspecies need not be totally isolated from one another, e.g. if part of a cline. However, most subspecies are separated by barriers. Isolated thus, subspecies may continue to diverge and their ability to interbreed may decrease as successive mutations alter the genotype to suit local environmental conditions. When both populations have become so distinct that they can no longer produce viable, self-perpetuating offspring when reunited, each is regarded as a true species. This appears to have happened to Acherontia atropos and Acherontia styx.

The time factor in this process of evolution is very important. If only a short period elapses between separation and reunification, the differences between subspecies would become submerged by an intermingling of their genes. This might take some time and, until completed, numerous hybrid populations would exist at various stages of introgression. Often secondary clines, with either a narrow or wide suture-zone containing many intermediate hybrids, are produced in this way, e.g. such as between Sphinx pinastri and Sphinx maurorum in the Pyrenees and southern France. This process also appears to have happened with the various subspecies and races of Clarina kotschyi, Hyles euphorbiae, Hyles tithymali, Hyles nicaea and Deilephila porcellus in the western Palaearctic.

When a much longer separation has occurred, reunification may no longer allow interbreeding due to the number of accumulated differences between the 'subspecies', such as between Acherontia atropos and Acherontia styx, Sphingonaepiopsis gorgoniades and Sphingonaepiopsis kuldjaensis, and Hyles hippophaes and Hyles chamyla.

Unfortunately, unless there is an overlap in ranges and no introgression, two physically isolated and visually distinct populations cannot be positively identified as true species. The western Palaearctic Smerinthus ocellatus and the Oriental Smerinthus planus appear, as phenotypes, to be two valid species. However, in Europe, a virgin female Smerinthus planus will usually attract some males of Smerinthus ocellatus during the right season. They pair successfully and the offspring are viable, although of reduced vigour. Is Smerinthus planus merely a subspecies of Smerinthus ocellatus?

The relationship between Hyles lineata and Hyles livornica was questioned for many years until Harbich (1980a, 1982) proved them to be distinct -- or did he? Certainly, crosses between North African and American moths produce non-viable offspring over three generations, but what of hybrids between Oriental, Australian and American material? With two distinct species at either end, could not the Hyles lineata/Hyles livornica complex be a cline? Under certain circumstances the terms 'species' and 'subspecies' must therefore often be used in a subjective fashion. Such is also the case with the Hyles euphorbiae/Hyles tithymali complex.


ECOLOGICAL INFLUENCES ON SPECIATION

Another element driving speciation is ecology. A basic factor governing the interactions of species which make up an ecosystem is competition; two species cannot permanently occupy the same ecological niche. Thus competition leads either to the departure or extinction of one of the competitors or, more commonly, a shift in the genetic make-up of one (or both) species so that neither has identical ecological requirements.

The genus Smerinthus illustrates this well. All species utilize Salix as their main hostplant. As this plant genus is distributed across the entire western Palaearctic, this is the potential regional range of all local species of Smerinthus, a genus of Holarctic origin. However, in order to exploit separate niches and avoid competition, each has adapted to a certain range of climatic conditions: Smerinthus caecus can tolerate short, cool summers and very cold, dry winters, hence its confinement to the northern boreal forests; Smerinthus kindermannii can tolerate long, hot, dry summers and short, cold, dry winters, thus its south-easterly distribution; Smerinthus ocellatus can exploit intermediate ranges including cool, moist winters. There are areas where Smerinthus ocellatus does overlap in range with the other two species, but closer investigation reveals that local environmental conditions separate them. In Asia Minor Smerinthus kindermannii usually occupies hot valley floors, and Smerinthus ocellatus the cooler mountains (Pittaway, 1993).

To retain its genetic individuality (because this may confer some ecological advantage), a species or 'sibling species' usually possesses mechanisms to prevent interbreeding with related species. This is known as 'reproductive isolation' and many factors are involved. These can be divided into two groups, pre-copulatory and post-copulatory mechanisms.

Pre-copulatory mechanisms reduce the incidence of unviable matings and wastage and/or dilution of genetic material, and enhance reproductive capacity. Several are recognized:


    • Spatial isolation. Two closely related species occupy two separate geographical regions and never meet. This appears to be the case with the eastern Palaearctic Smerinthus planus and Smerinthus ocellatus from the western Palaearctic.

    • Temporal isolation. Individuals of two species emerge at different times of the year, e.g. Hyles euphorbiae and Hyles vespertilio.

    • Sexual isolation. Females of each species have sex pheromones which vary in composition. The male genitalia in some genera are very similar, e.g. Hyles, and would in themselves prove an ineffective barrier.

    • Mechanical isolation. This consists principally of differences in male genitalia. Whilst not guaranteeing reproductive isolation, when coupled with other mechanisms, this process is effective.

    • Behavioural isolation. Females of one species 'call' and mate at a different time from females of closely related species. This form of temporal isolation appears to be the main mechanism used to keep Hyles euphorbiae, Hyles tithymali and Hyles gallii apart. This same mechanism is used by closely related species of Callosamia (Saturniidae) in North America.

    • Ecological isolation. Gene flow between 'species' can decrease following genetic linkage between ecologically important traits in sex chromosomes. This may be a secondary factor bringing about the evolutionary separation of Hyles euphorbiae and Hyles tithymali in North Africa.


Post-copulatory mechanisms are what might be called 'mechanisms of last resort'. These include the following:


    • Zygote mortality. The genetic material within the two contributing gametes is incompatible. The zygote dies.

    • Lack of hybrid vigour. The offspring die before reaching adulthood. This is the fate of most hawkmoth hybrids. Most larvae form up and die within the egg.

    • Adult sterility. Gametes produced by the hybrid imago may have either an incomplete structure, or one which is incompatible with either parent.

    • Hybrid breakdown. The F1 generation is fully or partially fertile, but each subsequent cross has successively less vitality and ultimately the offspring are sterile. Hybrids of the interspecific cross Hyles lineata x Hyles livornica, when paired with each other, are 95 per cent viable (Harbich, 1980a, 1982). Their offspring, the F2 generation, produce ova of which only 1--6 per cent hatch. Genitalia may also atrophy, as in the cross Laothoe populi populi x Laothoe austauti.


Although behavioural and genetic mechanisms operate to maintain species, the main factors that fragment or isolate populations leading to speciation are physical or environmental.


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