STRUCTURE OF THE WING-VEINS 



121 



The wings, then, are simple, very thin chitinous lamellate expan- 

 sions of the integument, which are supported and strengthened by 

 an internal framework of hollow chitinous tubes. 



The veins. The so-called " veins " or " nervures," which are situ- 

 ated between the upper and under layers of the wing are so disposed 

 as to give the greatest lightness and strength to the wings. Hagen 

 has shown that in the freshly formed wings these two layers can be 

 separated, when it can be seen that the veins pass through each layer. 



These veins are in reality quite complex, consisting of a minute 

 central trachea enclosed within a larger tube which at the instant the 

 insect emerges from the nymph, or pupa, as the case may be, is filled 

 with blood (Fig. 136). Since these tubes at first contain blood, which 

 has been observed to circulate through them, and since the heart can 

 be most easily injected through them, they may more properly be 

 called veins than nervures. The shape and venation of the wings 

 afford excellent ordinal as well as family and generic characters, 

 while they also enable the systematist to exactly locate the spots 

 and other markings of the wings. The spaces enclosed by the veins 

 and their cross-branches are called cells, and their shape often affords 

 valuable generic and specific characters. 



The structure of a complete vein is described by Spuler. In a 

 cross-section of a noctuid moth (Triphcena pronuba, Fig. 136) the 

 chitinous walls are seen to consist of two layers, an outer (u) and 

 inner (c), the latter of which takes a stain and lies next to the 

 hypodermis (%). In the cavity of the vein is the trachea (tr), 

 which shows more or less distinctly the so-called spiral thread; 

 within the cavity are also Semper's " rib " (r) and blood-corpuscles 



FIG. 136. Cross-section of wing FIG. 137. Cross-section of wing of Pieris : *, insertions 

 of Pronuba. After Spuler. of scales. After Spuler. 



(6c), which proves that the blood circulates in the veins of the com- 

 pletely formed wing, though this does not apply to all Lepidoptera 

 with hard mature wings. We have been able to observe the same 

 structure in sections of the wing of Zygsena. 



