122 REPORT — 1860. 



entered upon, and the question asked, whether it was possible in the present state 

 of our knowledge of the subject to arrive at any explanation of them. It was 

 remarked that the only hypothesis yet put forward which would explain these laws, 

 was that of " genetic relationship '' between species or their descent from a common 

 ancestral form. It was an acknowledged fact that the best naturalists were at 

 issue as to the precise limits between representative species and local varieties. 

 It was generally allowed that the latter were descendants of a common progenitor, 

 and the reasons given for the belief in the case of ''local varieties " might be shown 

 to be equally applicable to " representative species." Specific differences being once 

 granted to have originated from natural causes, it would be impossible to stop here, 

 and it must follow that greater divergences may have resulted from the operation 

 of similar agents acting through longer periods of time. 



On some Peculiar Forms amongst the Micro-Lejndopterous Larva. 

 By H. T. Stainton, F.L.S. 



It is well known that the normal form of a Lepidopterous larva is a cylinder, flat- 

 tened beneath, and slightly tapering and rounded at each end. To this, the typical 

 form of a Lepidopterous larva, we have abundant exceptions in most groups; thus 

 we have the woodlouse-shaped larva? amongst the butterflies, and again amongst 

 the Bombycina ; and in the latter group we have also numerous instances of larvae 

 adorned with humps or large protuberances on several of the segments, and in some 

 of the Noctuae larvae we observe a protuberance on the eleventh segment. 



The normal number of legs is sixteen, that is, six true legs and ten prolegs ; but 

 two of the latter are wanting in some of the Bombycina and in some of the Noctuina, 

 and in the whole group of the Geometrina from four to six of the prolegs are 

 wanting. 



In the group of the Torticina there are very few deviations from the typical form 

 of the larva ; but amongst the Tineina we find many genera which give instances 

 of very considerable deviation from the regular cylindric form. 



Among the most curious forms in this family, the larvae of the genus Phyllocnist in 

 may be mentioned ; in these the hinder extremity is so drawn out that it reminds 

 us of the rat-tailed larvae amongst the Diptera, though the object of the prolonged 

 tail in the Phyllocnistis larvae is very different. These larvae are also perfectly 

 apodal, and the structure of the mouth is peculiar: the jaws of all other Lepido- 

 pterous larvae terminate in two sharp-pointed mandibles ; the mandibles of a Phyl- 

 locnistis are perfectly blunt and rounded, like the points of lace-scissors. The 

 reason of this singular formation is pretty evident ; the larvae of the genera Coleo- 

 phora and Lithocolletis, which mine in the interior of leaves, feed on the parenchyma, 

 which they detach piece by piece hy their sharp mandibles and swallow ; but the 

 larvae of Phyllocnistis, though feeding beneath the cuticle of the leaf, do not eat the 

 parenchyma, and a leaf eaten by one of these larvae, if held up to the light, shows 

 no trace of the attacks of the larva. On what then do they subsist ? The larvae 

 mine rather rapidly forwards beneath the cuticle, raising the cuticle from the epi- 

 dermis, and they apparently devour something which they find between the two, 

 which, as they do not seem to remove any solid matter of the leaf, must be of a 

 juicy nature. It is no doubt essential to the comfort of these larva? that the cuticle 

 should not remain detached from the parenchyma in those parts of the leaf which 

 the larva has passed over, and accordingly we find that the cuticle again becomes 

 attached to the parenchyma immediately behind the larva, and that the cuticle 

 may be let down gradually and gently is, I believe, the cause of the prolonged 

 attenuated tail. The object of the blunt mandibles, in like manner, appears to be 

 to avoid any risk of the larva piercing the cuticle, which by letting in the external 

 air would probably be fatal to the existence of the larva, as these larvae have to 

 move their jaws in constant juxtaposition to the cuticle, which, in the aspen tree 

 (which is frequented by the commonest species of the genus, P. sitffusella), is re- 

 markably thin ; it must be a great convenience to the larva that the structure of its 

 jaws is such that it can eat its fill without any danger of piercing the cuticle. 

 Sharp-pointed jaws are necessary to a larva which feeds on the harder parts of 

 leaves ; but to this, which only, as it were, sucks up the juice, sharp-pointed jawa 

 are quite unnecessary. 



