Experiments have been made on pupae by Merrifield and others, with the result of showing that by changes of temperature applied at certain moments some of the colours or marks of the butterfly that will emerge can be altered.
It is found that in certain localities the colour of various kinds of butterflies more or less agrees, while it differs from that of the same butterflies found in other localities. Thus Weir speaks of a duskiness common to various butterflies in Java, and calls it "phaeism"; and Bates states that in the Amazon valley numerous species of butterflies vary in a similar manner, as regards colour, in a locality. This phenomenon is now called "homoeochromatism," and is supposed to be due to the effect of local conditions on a susceptible organisation, though there is no experimental evidence of this.
Mimicry.—There are many cases in Lepidoptera of species that depart more or less strongly in appearance from those forms to which they are considered to be allied, and at the same time resemble more or less closely species to which they are less allied. This phenomenon is called mimicry.[[204]] Usually the resembling forms are actually associated during life. Bates, who observed this phenomenon in the Amazon valley, thought that it might be accounted for by the advantage resulting to the exceptionally coloured forms from the resemblance;[[205]] it being assumed that these were unprotected, while the forms they resembled were believed to be specially protected by nauseous odours or taste. It was, in fact, thought that the destroying enemies were deceived by the resemblance into supposing that the forms that were in reality edible were inedible. This subject has been greatly discussed, and in the course of the discussion numerous cases that could not be accounted for by Bates's hypothesis have been revealed. One of these is the fact that resemblances of the kind alluded to very frequently occur amongst inedible forms. This also has been thought to be accounted for by a supposed advantage to the Insects; it being argued that a certain number of "protected" forms are destroyed by enemies the instincts of which are faulty, and which therefore always require to learn by individual experience that a certain sort of colour is associated with a nasty taste. The next step of the argument is that it will be an advantage to a protected butterfly to form part of a large association of forms having one coloration, because the ignorant enemies will more easily learn the association of a certain form of coloration with nastiness; moreover such destruction as does occur will be distributed over a larger number of species, so that each species of a large, similarly coloured, inedible association will have a less number of its individuals destroyed. It is scarcely a matter for surprise that many naturalists are very sceptical as to these explanations; especially as the phenomena are supposed to have occurred in the past, so that they cannot be directly verified or disproved. It has not, however, been found, as a matter of fact, that even unprotected butterflies are much destroyed in the perfect state by birds. Moreover, in endeavouring to realise the steps of the process of development of the resemblance, we meet with the difficulty that the amount of resemblance to the model that is assumed to be efficient at one step of the development, and to bring safety, is at the next step supposed to be inefficient and to involve destruction. In other words, while analysis of the explanation shows that it postulates a peculiar and well-directed discriminative power, and a persistent selection on the part of the birds, observation leads to the belief that birds have been but little concerned in the matter. If we add to this that there is no sufficient evidence that the species now similar were ever dissimilar (as it is supposed they were by the advocates of the hypothesis), we think it is clear that the explanation from our point of view is of but little importance.[[206]] The comparatively simple, hypothetical explanation, originally promulgated by Bates, is sometimes called Batesian mimicry; while the "inedible association" hypothesis is termed Müllerian mimicry.
There is one branch of the subject of mimicry that we think of great interest. This is the resemblance between Insects of different Orders; or between Insects of the same Order, but belonging to groups that are essentially different in form and appearance. It is not infrequent for beetles to resemble Hymenoptera, and it is still more frequent for Lepidoptera to resemble Hymenoptera, and that not only in colour and form, but also in movements and attitude. Druce says: "Many of the species of Zygaenidae are the most wonderful of all the moths; in some cases they so closely resemble Hymenoptera that at first sight it is almost impossible to determine to which Order they belong."[[207]] W. Müller says: "The little Lepidoptera of the family Glaucopides, that are so like certain wasps as to completely deceive us, have when alive exactly the same manner of holding their wings, the same restless movements, the same irregular flight as a wasp."[[208]] Seitz and others record a case in which a Brazilian Macroglossa exactly resembles a humming-bird, in company with which it flies; and the same naturalist also tells us[[209]] of a Skipper butterfly that greatly resembles a grasshopper of the genus Tettix, and that moreover makes movements like the jumping of grasshoppers. In most of these cases the probabilities of either original similarity, arrested evolution, or the action of similar conditions are excluded: and the hypothesis of the influence, by some means or other, of one organism on another is strongly suggested.
The classification of Lepidoptera was said by Latreille a century ago to be a reproach to entomologists. Since that time an enormous number of new species and genera have been described, but only recently has much advance been made in the way of improvement of classification. The progress made has been limited to a better comprehension and definition of the families. The nervuration of the wings is the character most in vogue for this purpose. As regards the larger groups, and Phylogeny, there is a general opinion prevalent to the effect that Micropterygidae, Eriocephalidae and Hepialidae are in a comparatively primitive condition, but as to the relations of these families one with the other, or with other Lepidoptera, there is a wide difference of opinion.
Fig. 176—Clubs of butterflies' antennae. Terminal portions of antenna of, 1, Pieris brassicae; 2, Styx infernalis; 3, Hestia idea (sub-family Danaides); 4, Eudamus proteus, and 5, Limochores taumas (Hesperiidae). (After Schatz and Scudder.)
The primary divisions of the family most often met with in literature are:—either Rhopalocera (= butterflies) and Heterocera (= moths); or Macrolepidoptera and Microlepidoptera; the Macrolepidoptera including the butterflies and large moths, the Microlepidoptera being limited to the families Tineidae (now itself in process of division into numerous families) and Tortricidae; some entomologists including also Pyralidae, Pterophoridae and Orneodidae in Microlepidoptera. The division of all Lepidoptera into two series is merely a temporary device necessitated by imperfect acquaintance with morphology. The division into Macro- and Micro- lepidoptera is entirely unscientific.
Series 1. Rhopalocera or Butterflies.—Antennae knobbed at the tip or thickened a little before the tip, without pectinations, projecting processes, or conspicuous arrangements of cilia. Hind wings without a frenulum, but with the costal nervure strongly curved at the base (Fig. 161, II, B).
Series II. Heterocera or Moths.—Antennae various in form, only rarely knobbed at the tip, and in such cases a frenulum present. In the large majority a frenulum is present, and the costal nervure of the hind-wing is either but little arched at the base (as in Fig. 161, I, B) or it has a large area between it and the front margin; but in certain families the hind wing is formed much as in Rhopalocera.