mimetic Lepidoptera. The first is the occurrence of mimicry
only in the female sex. The reason for this is to be found in
the greater need of protection of the female which is slower in flight
than the male and is exposed to special danger of attack when resting
to lay her eggs. The second noteworthy phenomenon is the mimicry
of more than one protected species by members of a single
species. This is a not uncommon occurrence, and in the case of
Batesian mimicry the explanation is probably this. When an
edible species gains protection by mimicking a distasteful one, there
is a likelihood of its increasing in numbers until it equals or surpasses
its model in this respect. Were this to take place the purpose of
the mimicry would be abortive, because enemies would probably
not refrain from slaughter if even every alternate capture proved
palatable. It is advantageous therefore that the numbers of the
mimetic species should be fewer than those of the model; and this
appears to be achieved in some cases by the individuals of the mimetic
species dividing themselves between two or more models.
Spiders furnish numerous instances of mimicry. Though simple in kind, many of these are as perfect illustrations of the phenomenon as any found in the animal kingdom.
Amongst the orbweavers of the family Argyopidae there are species belonging to the genera Cyclosa and Cyrtophora which closely resemble small snail-like gastropods as they cling to the underside of leaves with their legs drawn up. Other members of the same family—like Araneus coccinella, and Paraplectana thorntoni—imitate beetles of the family Coccinellidae which are known to be distasteful; and certain genera of the family Salticidae (Homalattus and Rhanis) closely resemble small hard-shelled beetles.
The most perfect cases, however, are exhibited by those species which imitate ants. The structural modifications required to convert a spider into the image of an ant are of a more complicated character than those that serve the same purpose in an insect. All insects have the same regional division of the body into head, thorax and abdomen, the same number of legs, a pair of antennae and a segmented abdomen. Spiders on the contrary have no antennae, no separate “head,” an unsegmented abdomen and an additional pair of legs. In the majority of ant-imitating spiders the forepart of the cephalothorax is constricted on each side to resemble the neck of the insect, and in many cases the similarity is increased by the presence of a stripe of white hairs which has the optical effect of cutting out an extra piece of integument, exactly as occurs in analogous cases in insects. Narrowing of the posterior portions of the spider's cephalothorax and sometimes of the anterior end of the abdomen reproduces the slender waist of the ant, and frequently transverse bands of hairs represent the segmentation of this region in the insect. The legs become slender and those of the first or of the second pairs are held up and carried in front of the head to simulate the antennae of the ant. Added to this the spiders commonly copy to the life the mode of progression and the restless activities of their models.
The likeness presented varies considerably in degree from a general resemblance to several species, such as is seen in the Salticid spider (Peckhamia picata) of North America, to a close similarity to particular species. To this category belong Myrmarachne plataleoides, one of the Salticidae, and Amyciaea forticeps, one of the Thomisidae which in India imitate and live with the vicious little red ant (Oecophylla smaragdina); also Myrmarachne providens, which mimics the red and black Indian ant (Sima rufonigra); and the South American species of Clubionidae, e.g. Myrmecium nigrum, which is an accurate copy of the large black ant (Pachycondyla villosa).
Sometimes it is only the males of a species of spider that mimic ants, as in the case of Ildebaha mutilloides and I. myrmicaeformis, two South American species of the family Argyopidae, in which the females are protected by strong spine-armature. The males are without these protective spines and are exposed to special dangers as they wander in search of the webs of the females. In South Africa too the males of a species of Eresidae (Seothyra) resemble and are found in company with a large ant (Camponotus fulvopilosus), which is common on the veld. Like the males of Ildebaha, those of Seothyra, wander about by day in search of the females which live concealed in burrows. Many other spiders belonging to the Theridiidae and Linyphiidae also mimic ants; but it is needless to enumerate them, the most perfect examples of this phenomenon being found in the families Clubionidae and Salticidae.
Ant-mimicking spiders have been seen now and again to devour their models. It has therefore been suggested by some and taken for granted by others that the resemblance comes under the category of aggressive mimicry and that the ants are deluded by this resemblance into regarding the spiders as members of their own species. That the ants do not destroy them is certain; but that they are deceived by the superficial similarity of the spiders to themselves is highly improbable, for these insects are capable of distinguishing a strange ant belonging to the same species if it comes from another colony. Moreover, the above-suggested explanation does not coincide with the explanation of the likeness to ants shown by certain insects such as Myrecophana fallax, the ant and leaf-like Membracid Homopteron and the larvae of the lobster-moth (Stauropos fagi), which are plant-eaters. It is probable that one explanation—namely, that of protection—covers all cases of ant-mimicry; and this explanation lies in all probability in the immunity from the attacks of most insectivorous enemies that ants enjoy, and especially from predaceous wasps of the family Pompilidae which annually destroy thousands upon thousands of spiders to feed their larvae; and since more than one observer has testified to the fear and abhorrence these wasps have of ants, it is needless to look farther for the benefit ant-mimicry is to spiders. These wasps, moreover, also provision their nurseries with caterpillars, grasshoppers and other insects. Hence it may be inferred that the insects which imitate ants profit in the same way that spiders do from this form of mimicry.
In the above-cited historical instance of mimicry amongst some South American Lepidoptera which formed the foundation of Bates theory, species of butterflies, belonging to the Ithomiine genus Itura and the Danaine genus Thyridia, both unpalatable forms, resemble each other. This is a very simple case of the possession of the same type of coloration by two or more protected insects inhabiting the same district. The significance of this phenomenon, as already stated, was first explained by Fritz Müller; but although the term “Müllerian mimicry” has been assigned to this and similar instances, they are not strictly speaking cases of mimicry at all but of warning coloration. Poisonous or noxious animals usually have some special advertising attribute, sometimes the display of conspicuous coloration, as in the skunk; sometimes the emission of sound as in the rattlesnake; sometimes a combination of the two, as in the common porcupine and the large black scorpions of Africa and India. Such characters have been termed by Professor Poulton “aposematic.” Neither of the above-mentioned animals is mimicked; but where two or more noxious animals, inhabiting the same district, resemble each other, both being aposematically or warningly coloured, the likeness is said to be “synaposematic.” Synaposemasy is Müllerian mimicry. Finally, the likeness of an edible species to a warningly coloured inedible one in the same locality is termed “pseudaposematic,” in allusion to the pretentiousness or falsity of the warning signal. Pseudaposemasy is Batesian mimicry.
An important phenomenon connected with insect mimicry is the convergence of several species in the same area towards a common type of coloration and shape, exhibited by one or more than one protected form. The resemblance shows various grades of completeness; and the convergent mimics may be themselves noxious, or edible and innocuous. In other words the insects entering into the combination may furnish instances of Batesian and of Müllerian mimicry. Very commonly different species of aculeate Hymenoptera, inhabiting the same district, form the centres of mimetic attraction for insects of various orders, so that a considerable percentage of the insect-fauna can be arranged in groups according to the pattern of the particular model the species have copied. Good illustrations of this law have been discovered by Guy Marshall in Mashonaland. He found on the same day on a bud of vetch, specimens of black ants (Camponotus sericeus and C. cosmicus), black ant-like Hemipterous insects (Megapetus atratus), and the ant-like Orthopteron (Myrmecophana fallax) (cf. supra). In this little coterie the ants are beyond question the models towards which the bug and the grasshopper have converged in appearance. Since many of the insects of the order Hemiptera are distasteful, the mimicry of the bug (Megapetus) is in this case probably Müllerian or synaposematic; the grasshopper (Myrmecophana), on the other hand, is probably edible and the mimicry is Batesian or pseudaposematic. This is a simple case consisting of a small number of component species. Others are more complex, numerous species being involved. In Mashonaland, for instance, a large number of genera and species of Hymenoptera belonging to the Apidae, Eumenidae, Sphegidae, Pompilidae, Scoliidae, Tiphiidae and Mutillidae, resemble each other in having black bodies and dark blue wings. The same style of coloration is found in Coleoptera of the families Cetoniidae and Cantharidae; in Diptera of the families Asilidae, Bombylidae, Tabanidae and Tachinidae; in Hemiptera of the family Reduviidae and in Lepidoptera of the family Zygaenidae. In this instance the Hymenoptera, of which the coloration is synaposematic, form together a composite model which the other insects have mimicked. Of the latter, the Lepidopteron (Tascia homochroa) is distasteful, as also are the beetles of the family Cantharidae (e.g. Lytta moesta). Probably the bugs too (e.g. Harpactor tristis) are protected. The mimicry of these insects therefore is synaposematic; but some, at all events, of the flies like the Bombylid Exoprosopa umbrosa, probably form pseudaposematic elements in the group. Into another category Hymenoptera enter not as models but as mimics, the models being inedible Malacodermatous beetles mostly belonging to the genus Lycus and characterized by orange coloration set off by a large black patch upon the posterior end of the elytra and a smaller black spot upon the thorax. Towards this Lycoid centre have converged Coleoptera (beetles) of the sub-order Lamellicornia (Copridae), Phytophaga; Heteromera (Cantharidae) and Longicornia; Hemiptera of the families Pyrrhocoridae, Lygaeidae, and Reduviidae; Lepidoptera of the families Arctiidae and Zygaenidae; Diptera of the family Asilidae; and lastly Hymenoptera of the families Braconidae, Pompilidae, Crabronidae and Eumenidae. With the exception of the Asilid fly and perhaps some of the Longicorn and Phytophagous beetles, which are probably protected Batesian mimics, all the other species constituting the above-mentioned assemblage are, it is believed, Müllerian or synaposematic mimics. In the three cases
