TRACE EAT A. 



339 



thoracic appendages are the first to be formed, and the second pair of 

 maxillae makes its appearance before the other cephalic appendages. 



The history of the changes in the embryonic appendages during the 

 attainment of the adult condition is beyond the scope of this treatise, but 

 it may be noted that the second pair of maxillae are relatively very large 

 in the embryo, and not infrequently (Libellula, etc.) have more resemblance 

 to the ambulatory than to the masticatory appendages. 



The exact nature of the wings and their relation to the other segments 

 is still very obscure. They appear as dorsal leaf-like appendages on the 

 2nd and 3rd thoracic segments, and are in many respects similar to the 

 tracheal gills of the larvae of Ephemeridae and Phryganidae (fig. 188 A), of 

 which they are supposed by Gegenbaur and Lubbock to be modifications. 

 The undoubtedly secondary character of the closed' tracheal system of larvae 

 with tracheal gills tells against this view. Fritz Miiller finds in the larvae 

 of Calotermes rugosus (one of the Termites) that peculiar and similar dorsal 

 appendages are present on the two anterior of the thoracic segments. 

 They are without tracheae. 



The anterior atrophies, and APR 



the posterior acquires tra- 

 cheae and gives rise to the 

 first pair of wings. The 

 second pair of wings is 

 formed from small processes 

 on the third thoracic seg- 

 ment like those on the other 

 two. Fritz Miiller concludes 

 from these facts that the 

 wings of Insects are deve- 

 loped from dorsal processes 

 of the body, not equivalent 

 to the ventral appendages. 

 What the primitive function 

 of these appendages was is 

 not clear. Fritz Miiller sug- 

 gests that they may have 

 been employed as respiratory 

 organs in the passage from 

 an aqueous to a terrestrial 

 existence, when the Ter- 

 mite ancestors lived in moist 

 habitations a function for 

 which processes supplied 

 with blood-channels would 

 be well adapted. The un- 

 doubted affinity of Insects 

 to Myriapods, coupled with 



FIG. 188. FIGURES ILLTJSTKATING AQUATIC RE- 

 SPIRATION IN INSECTS. (After Gegenbaur.) 



A. Hinder portion of the body of Ephemera 

 vnlgata. . longitudinal tracheal trunks; b. ali- 

 mentary canal; c. tracheal gills. 



B. Larva of Jilschna grandis. a. superior 

 the discovery by Moseley of longitudinal tracheal trunks; 6. their anterior end; 

 a tracheal system in Peri- c. portion branching on proctodseum ; o. eyes. 



C. Alimentary canal of the same larva from the 

 side. a. b. and c. as in B; d. inferior tracheal 

 trunk; e. transverse branches between upper and 



sects can have sprung di- lower tracheal trunks. 



222 



patus, is however nearly 

 fatal to the view that In- 



