32 BRITISH LEPIDOPTERA. 



The lepidopterous larva, by its active, independent existence, under- 

 goes special modification and development, in order to protect itself 

 from its various enemies. Hence the larvae of different species assume, 

 by modification, a manifold variety of shapes, and of arrangement of 

 the various external structures hairs, tubercles, etc. As the most 

 specialised larvae present, therefore, such wide divergences from the 

 original type from which they have sprung, it becomes necessary for 

 us often to homologise the complex structures which they now bear 

 with the simple structures from which they originated, and to do this 

 a comparison must be instituted with those larvas which, from the 

 exigencies of their environment, are but little changed from the more 

 ancestral larvae. The newly-hatched larvae of many species, which 

 are very specialised in their adult stages, have the specialised structures 

 in a very simple condition, both as to form, structure and arrange- 

 ment ; whilst many boring and case-bearing larvae are still more 

 simple in the structure and arrangement of the tubercles, hairs and pro- 

 legs, which are especially prone to be changed by external conditions. 

 Larvae which show this simple arrangement of tubercles, hairs and 

 prolegs, are often spoken of as generalised, in contradistinction to those 

 in which the structures are complicated, and which are termed 

 specialised, larvae. It must not be forgotten, however, that the most 

 generalised of all lepidopterous larvae must be far in advance of the 

 larvae of those insects (Orthoptera, etc.), with incomplete metamor- 

 phoses. There can be little doubt that insects belonging to these 

 orders, in which the metamorphoses are carried on within the very 

 narrowest limits, and in which the various stages present but little 

 change, inter se y are much more ancestral than the insects belonging 

 to those orders in which the metamorphoses are distinct, and in which 

 the various stages bear but little resemblance to each other. 



The Lepidoptera which have the most generalised form of larvae 

 are the Eriocephalids, Micropterygids, Adelids, Tineids, Sesiids, 

 Psychids, Hepialids, Zeuzerids and the Tortricids. A comparison of 

 these with each other, and with larvae belonging to more specialised 

 super-families, soon gives us a clue as to the lines on which modifica- 

 tion has proceeded in the higher groups. 



Having glanced at the general structure of a lepidopterous larva, 

 we may deal with a few of the organs in more detail. 



The head of a caterpillar is divided into two lateral halves by a 

 suture, which divides, however, in the centre of the face, and leaves 

 between its forked branches a triangular space. This frontal triangle 

 is termed the clypeus, and is very often distinctly and characteristically 

 marked. Just within, and parallel to the central facial suture, is a 

 deeply-grooved furrow, which is the reverse of a ridge that faces in- 

 ternally, and to which the muscles of the head are attached. The true 

 sutural line is but little developed in newly-hatched larvae. Just 

 below the clypeus is a short inconspicuous piece of chitin, welded to the 

 clypeus. This is very distinct in some butterfly larvae (<?.//., the Papi- 

 lionids), and is known as the epistoma. To its lower edge, the usually 

 bi-lobed labrum or lip is attached by a fleshy hinge, enabling it to move 

 freely backwards and forwards upon the mandibles. These latter are 

 arranged on either side of the mouth ; each consists of a stout, 

 swollen, short, horny, plate, which is broader at the base, and becomes 

 somewhat pointed at the apex, which varies considerably in different 



