154 



INSECT TRANSFORMATION 



inpushed from the larval body-wall. The bud first arises as 

 a thickening of the epidermis, due to increasing depth of 

 certain cells ; these thickened cells form an enlarging region 

 which becomes folded on itself and enclosed in the pouch due 

 to the ingrowth of a sheet of thin cells from the skin. In close 

 association with the growing wing-bud is a tracheal trunk, 

 from which air-tubes grow into the space between the bud's 

 two folds. The arrangement of these tubes foreshadows the 

 plan of nervuration in the developed wing, for the sub-costal, 

 radial, cubital and anal systems of longitudinal nervures or at 

 least some of them are represented by the growing air-tubes 

 (Fig. 87). It is remarkable that while a lacewing, a butterfly 

 or a moth-pupa has a wing with the full set of tracheal trunks, 

 in the pupal wing of a scorpion-fly or a caddis-fly only two or 



FIG. 88. WING OF PUPAL ALDER-FLY (Sialis). 



Showing tracheal tubes and pale bands which indicate course 

 of nervures (Sc, subcostal ; R, radial ; M, median ; Cu, cubital ; 

 A, anal). X5- After Comstock, " Wings of Insects". 



three trunks are apparent. As during the final larval stages, 

 the pupal body begins to take shape and the wing-buds are 

 thrust out from their pouches, narrow pale bands (Fig. 88) 

 can be distinguished forming tracks as it were in which the 

 air-tubes lie ; these tracks show where the thickenings of the 

 wing-cuticle that form the completed nervures are becoming 

 defined. 1 Within these thickenings which assume a more or 

 less cylindrical form around long blood-spaces, the air-tubes 

 become enclosed, while over the rest of the area of the develop- 

 ing wing the upper and lower surfaces become approximated, 

 so that when the cuticle has dried and hardened an apparently 

 simple and firm membrane is provided for the purpose of 

 flight. 



1 J. H. Comstock : " The Wings of Insects ". Ithaca, New York, 1918. 

 R. J. Tillyard : " The Panorpoid Complex ". Pt. Ill, " The Wing- Venation", 

 Proc. Linn. Soc., N.S.W., XLIV. (pt. Ill), 1919. 



