CAUSES OF HYPERMETAMORPHISM 707 



larval Crustacea, the thoracic and abdominal appendages do not arise 

 until some time after hatching from the egg. 



8. While cases of the suppression or abbreviation of larval char- 

 acters and direct development are not uncommon in echinoderms 

 and crustaceans, in insects this phenomenon occurs only so far as 

 yet known in the Diptera. In these insects the polypody in the 

 embryo is outgrown, or lost, the embryos and larvae not having even 

 the temporary rudiments of abdominal appendages. The campo- 

 deoid characters also are entirely suppressed, dropped, or lost in the 

 more specialized holometabolous orders, Lepidoptera, Hymenoptera, 

 and Diptera, though retained in the more primitive and generalized 

 Coleoptera. (This proves that the Coleoptera are lower or more 

 primitive and generalized than the other orders mentioned.) This 

 abbreviation or loss of organs is, as Hyatt and Arms claim, due to 

 the prepotency of acquired characters in phylogeiiy, and are also the 

 result of homochronous heredity. 



" The Insecta of the more specialized orders, x.-xvi., afford, next to some para- 

 sites, the most notable examples of this mode of evolution. Their larval or 

 nepionic, and pupal or neanic, stages are prolonged at the expense of the 

 ephebic, winged stage, and the reasons for this prolongation are found in the 

 great number of new features introduced into these stages of development in these 

 orders as contrasted with those of the more primitive, and, in large part, more 

 ancient orders, i.-ix. The law of tachygenesis has been at work here, as in the 

 former cases alluded to above, and it is shown in the encroachments of the 

 adaptive characteristics of the caterpillar, grub, and maggot upon the inherited 

 characteristics of the Thysanuran stage, which loses its ancestral characteristics, 

 until in most cases they are either obsolete or recognizable with difficulty." 

 (Hyatt and Arms, Natural Science, 1896, p. 400.) 



9. In the holometabolous insects there is a resting, quiescent 

 stage during the pupal period, when the insect takes no food. In 

 this respect the more specialized insects differ from other metamor- 

 phic animals. The larva has an abundant supply of fat lasting 

 through pupal life, while in the quiescent pupa, respiration and cir- 

 culation is much lessened, the animal being as a rule motionless. 

 This resting stage is also necessary for the histolysis and forma- 

 tion of the adult body from the imaginal buds present in the larva. 



10. The hypermetamorphosis of Mantispa, Meloe, Stylops, etc., 

 indicate very plainly that the eruciform type of larva is derived 

 from the campodeoid, since one and the same insect passes through 

 these stages before reaching sexual maturity. 



11. As observed by Miall, the larva of insects differs from that 

 of other invertebrate animals in being larger than the adult. 



12. The metamorphoses of insects are in some important respects 



