30 BRITISH LEPIDOPTERA. 



and change of the colour of the egg are dealt with. In cases of par- 

 thenogenesis, the influence must be powerful enough to cause full 

 development, not only for one generation, but for one or more genera- 

 tions beyond the one normally reached, and in this way may be 

 explained the phenomenon that some species, which usually do not 

 multiply without sexual intercourse, occasionally produce partheno- 

 genetic young, even in cases like Sphinx liyiistri, Bombyx ntori, etc., 

 where it could scarcely be expected. It is remarkable that, in most 

 orders of insects, the parthenogenetic progeny is usually male, but, in 

 the Psychidae among Lepidoptera, helotoky, or the production of 

 parthenogenetic females, alone takes place. 



I may mention, in conclusion, that the great difference that exists 

 between parthenogenesis (1) in the Psychids, where it appears to 

 be, in some species, the rule rather than the exception, as it is in some 

 Cynipids and in bees (as regards male eggs), and (2) in all other Lepi- 

 doptera, where it is a rare and occasional phenomenon, is somewhat 

 striking and important. This difference, no doubt, is only one of 

 degree, but so great a degree as to be parallel to a difference in kind. 

 It is quite possible, too, by means of the Psychids, to ally the partheno- 

 genesis that takes place in Lepidoptera with that known to occur in 

 the Cynipidae, and the phenomena might perhaps be brought into 

 connection with a more primitive method of reproduction, e.g., gem- 

 mation. I am quite clear that the modus operandi of parthenogenesis 

 in Lepidoptera is still as obscure as ever, and that the explanation I 

 have offered does not help matters much. It, however, is the only 

 logical explanation that has occurred to me, and must be taken for, and 

 only for, what it is worth. 



CHAPTER V. 



THE EXTERNAL STRUCTURE OF THE LEPIDOPTEROU S LARVA. 



AT the time that the lepidopterous larva escapes from the egg, it 

 possesses true insect characters. Its body is composed of a series of 

 segments, containing the muscular, digestive, circulatory, respiratory, 

 and nervous systems. It breathes by means of tracheae, a series of 

 fine tubes composed of an elastic membrane, and kept open by a 

 spiral structure, which passes throughout their whole length. The 

 four segments of the head are now welded into an almost inseparable 

 whole, and, although the first three body-segments are assigned to the 

 thorax, there is no well-marked separation between the thoracic and 

 abdominal regions. The .skin of the newly-hatched larva is very soft, 

 but it quickly becomes harder, owing to the solidification of the horny 

 substance called chitin in the outer cuticle. Usually larvae have a 

 somewhat colourless skin when just out of the egg ; but the harden- 

 ing of the cuticle is frequently accompanied by the production of a 

 difference in colour, and by the development of the distinct markings 

 which are characteristic of the larval cuticle, so that an almost 

 colourless larva may, within an hour of hatching, become almost 

 black. This hardening does not affect the sutures, and the interseg- 



