Note 19 — Chapter IV

These groups of different species, some belonging to palatable and some to unpalatable groups, all exhibiting a close resemblance in colour and pattern, are far more strikingly developed in S. America than in either Asia or Africa, and it is not uncommon for eight or ten species to enter into such an association. A group of this sort which possesses unusual interest is the so-called "Transparency Group" from certain parts of the Amazon region. It was originally described by Bates with seven species belonging to six different genera. To-day it is said that no less than 28 species of this peculiar facies are known, though some are excessively rare. The majority are Ithomiines, but two species of the Danaine genus Ituna, the Pierine Dismorphia orise (Pl. XII, fig. 2), the Swallow-tail Papilio hahneli, and several species of diurnal moths belonging to different families (cf. Pl. XII, fig. 4) also enter into the combination. In connection with it there is a feature of peculiar interest in that the transparent effect is not always produced in the same way. In the Ithomiines such as Thyridia, where there are normally two kinds of scales, the wider ones for the most part lose their pigment, become much reduced in size and take on the shape of a stumpy V (Pl. XIV, fig. 3). Also they stand out for the most part more or less at right angles to the wing[19], and the neck by which they are joined to the wing membrane is very short. The longer and narrow form of scales also tend to lose their pigment and become reduced to fine hairs. In Dismorphia the scales, which are of one sort, are also reduced in size though apparently not in number. Like the wider scales of the Thyridia they tend sometimes to project at right angles to the wing membrane, though not to the same extent as in the Ithomiine: possibly because the neck of the scale is not so short. As in Thyridia these reduced scales lose their pigment except in the transition region round the borders of the transparent patches. In Ituna there is a difference. The scales are not reduced to the same extent in point of size. Their necks are longer as in normal scales and they lie flat on the wing membrane. The majority of the scales, as in the preceding cases, lose their pigment, but mixed up with them is a certain proportion, about one-quarter, in which the pigment is retained. In Castnia and in Anthomysa the scales on the transparent parts which are without pigment are also somewhat reduced in size, being stumpier than the normal ones. At the same time they tend to stand out at right angles to the wing membrane[20]. The neck here again is shorter in the transparent than in the pigmented scales. A good deal of stress has been laid upon this case by some supporters of the theory of mimicry, since it is supposed to shew that a similar effect can be brought about in a variety of ways; consequently the existence of this assembly of similar transparent forms belonging to various families cannot be put down as due to the effect of similar conditions, but must be regarded as having arisen in each instance in a different manner through the independent action of natural selection[21]. It is doubtful, however, whether such a conclusion necessarily follows from the facts. In all of the cases the process would appear to be similar: loss of pigment, reduction in the size of the scales, and eventually a tendency for the scales to stand at right angles to the wing—this last part of the process apparently depending upon the reduction of the neck of the scale. It has been said that greater transparency is brought about by the scales standing out at right angles in this way, but as the scales themselves are already transparent there would appear to be no reason why this should be so. Of course the process has not proceeded in all of the forms to the same extent. There is least change in Ituna where the scales are not much reduced in size and where a fair proportion are still pigmented. There is probably most in an Ithomiine such as Thyridia, where the scales are not only small and entirely without pigment, but also are for the most part neckless so that they stand out at right angles to the wing. Having regard to the fact that several widely separate genera with different types of scaling formed the starting points, the final results do not seem to preclude the supposition that the transparency has arisen through a similar process in all of them.