i > HI GIN OF Till-: TRACHEAE 553 



lengthy and interesting discussion, says that the question cannot 

 be, in the present state of our knowledge, solved. He, however, 

 seems inclined to believe that the prolegs are not merely secondary 

 structures, and that the rudiments of limbs may remain for a long 

 time in a latent state before their final development. Korschelt 

 and Heider are disposed to regard the abdominal appendages of 

 Lepidoptera and Hymeuoptera as true limbs, referring to Balfour's 

 statement that in the Crustacea there are different examples of the 

 loss sttid later appearance of limbs, such as the loss of the man- 

 dibular palpi of the zoea of decapods, and the loss in the zoe'a of 

 appendages in the Erichthus form of the Squilla larva correspond- 

 ing to the third pair of maxillipedes and first two pairs of legs of 

 Decapoda, and which are afterwards reproduced; similar cases 

 occurring in the Acarina. In the wasps and bees also, as is well 

 known, the imaginal disks of the thoracic appendages appear, the 

 legs themselves being suppressed in the larva (the imaginal disk 

 probably existing in an indifferent state), to reappear in the pupa 

 and imago. It does not, however, necessarily follow that the nu- 

 merous pairs of hooked ventral tubercles of certain dipterous larvae 

 (Ephydra) are true appendages. 



It seems to us that it is a strong argument for the view that these 

 prolegs are survivals of primitive limbs, that from similar embryonic 

 paired outgrowths on different segments arise the spring of Podu- 

 rans, the anal cerci, and three pairs of appendages forming the 

 ovipositor, and the anal legs of the Corydalus larva, as well as those 

 of caddis-worms ; at least five abdominal segments throughout 

 the class of insects as a whole bearing appendages in the adult. 



On the other hand the view of Haase, that the prolegs of cater- 

 pillars are secondary, adaptive characters, is supported by the fact 

 of the rapidity with which two pairs on the 3d and 4th segments 

 nearly disappear in the larvae of certain Noctuidae (Catocala, etc.), 

 a reduction evidently due to disuse. 



The tracheae. - - The tracheal system arises as ectodermal invagina- 

 tions on one side of the appendages, appearing soon after the latter. 

 The earliest condition of the tracheal invagination is seen in section 

 at Fig. 539, E, tr; as it deepens, it sends off diverticula or tracheal 

 branches, while the narrow mouth of the invagination forms the 

 stigma. The cup-like cavities sitiiated serially one behind the other, 

 and arising from the single tracheal invaginations, become at the 

 end or bottom of the cup elongated along the length of the body and 

 fused together at their ends; then the two longitudinal steins <>!' 

 the system arise, by a breaking through at the place where the 



