September, 1901] 



PSYCHE. 



249 



sected out, and sectioned (in any plane 

 desired) makes a detailed study of the 

 development of the wings and legs of 

 HoloiKsia (or any other large Tipulid), a 

 matter readily accomplished by any stu- 

 dent who has access to a microtome. 

 The limits of this paper preclude any 

 detailed account of the development. 

 For the orientation of students I figure 

 and briefly describe four stages (fig. 2) 

 in the development of a wing bud, 

 namely, a beginning stage, a middle 

 stage, a later stage, and a stage from a 

 larva nearly ready to pupate. These 

 four stages show all of the parts and the 

 most marked developmental changes of 

 the buds. They should be sufficient to 

 enable a student to interpret correctly 

 the parts and developmental phenomena 

 in a detailed study of the histology and 

 development of the wing-buds of any 

 insect with complete metamorphosis. 



In the first stage (fig. 2, A) figured 

 the hypoderm is thickened and invag- 

 inated and slightly folded. This folding 

 it will be noted results in a shallow 

 evagination within the cup or cavity 

 formed by the primary invagination. It 

 will be noted that the thickened (bud) 

 layer is perfectly continuous with the 

 normal hypoderm (true skin) which over- 

 lies the body just inside of the chitinous 

 cuticle. The neck of the bud is broad 

 and is filled by chitin. The hypoderm 

 layer composing the neck is not thick- 

 ened, or but slightly. This distinction 

 of thickening is later more emphasized, 

 the thin part being the peripodal mem- 

 brane, while the thickened part forms 

 the true wing-forming part of the bud. 



Just outside of the bud in the body 

 cavity and in immediate proximity to it 

 is a tracheal tube (cut transversely 

 across in the section) and giving off a 

 mass of fine tangled traiiicolcs which 

 tend to push into the concavity at the 

 base of the bud caused by the slight 

 evagination of the basal part of the bud. 

 These tracheoles are formed by a 

 peculiar enlargement and tubule-forming 

 of the cells of the epithelium of the 

 tracheal tube. The nuclei of these cells 

 are large and conspicuous. The trach- 

 eoles are simply fine capillaiy tubules, 

 and lack the spiral thread characteristic 

 of tracheae. 



In the next stage (fig. 2, B) figured 

 the folding is more pronounced, resulting 

 in a filling up of the cavity caused by 

 invagination, the neck of the bud is 

 narrower, and the distinction between 

 the peripodal membrane and the true 

 wing forming part of the bud layer 

 more pronounced. 'J'he chitin inside 

 the bud (which is of course perfectly 

 continuous with the actual outside of 

 the body) can be traced far down in the 

 bud forming a thin double layer indicat- 

 ing always the true external surface of 

 the developing wing. 



The third stage (fig. 2, C) shows the 

 two distinct layers of the wing enclosed 

 by the peripodal membrane, and the 

 long slender " neck " of the bud connect- 

 ing it with the normal skin hypoderm. 



The fourth stage (fig. 2, U) shows 

 the two wing layers in contact and the 

 forming veins (in cross section) along 

 this line of contact. The wing now lies 

 as a well formed wing pad with thick 



