122 TEXT-BOOK OF ENTOMOLOGY 



A cross-section of a vein of Picris brassicce shows that the large 

 trachea is first formed, and that it extends along the track between 

 the protoplasmic threads connecting the two hypodermal layers. 



The main tracheae throw off on both sides a number of secondary 

 branches showing at their end a cell with an iiitracellnlar tracheal 

 structure ; these accessory tracheae afterwards branch out. The acces- 

 sory or transverse tracheae often disappear, though in some moths they 

 remain permanently. Fig. 137 tr 2 represents these secondary veins 

 in the edge of the fore wing of Laverna vanella, arising from a main 

 trachea (tr) passing through vein I (v), two of the twigs extending to 

 the centre, showing that the latter has no homology with a true vein. 

 Only rarely and in strongly developed thick folds are the transverse 

 tracheae provided with a chitinous thickening, as for example in 

 Cossus ligniperda. Since from such accessory tracheae the transverse 

 veins in lepidopterous wings are developed, we can recognize in 

 them the homologies of the net-veins in reticulated venations. 

 There is no sharply defined difference between reticulated and non- 

 reticulated venations ; no genetic difference exists between the two 

 kinds of venation, since there occur true Blattidae both with and 

 without a reticulated venation (Spuler). 



In the fore wings of Odonata, Psocina, Mantispidge, and most 

 Hymenoptera is an usually opaque colored area between the costal 

 edge and the median vein, called the pterostigma. 



In shape the wings are either triangular or linear oval, and at the 

 front edge the main veins are closer together than elsewhere, thus 

 strengthening the wings and affording the greatest resistance to the 

 air in making the downward stroke during flight. It is noticeable 

 that when the veins are in part aborted from partial disuse of the 

 wings, they disappear first from the hinder and middle edge, those 

 on the costal region persisting. This is seen in the wings of 

 Embiidce (Oligotoma), Cynipidae, Proctotrupidee, Chalcids, ants, etc. 



The front edge of the wing is called the costal, its termination in 

 the outer angle of the wing is called the apex ; the outer edge (termen) 

 is situated between the apex and the inner or anal angle, between 

 which and the base of the wing is the inner or internal edge. 



\Yliile in Orthoptera, dragon-flies, Termitidae, and Neuroptera the 

 wings are not attached to each other, in many Lepidoptera they 

 are loosely connected by the loop and frenulum, or in Hymenop- 

 tera by a series of strong hooks. These hooks are arranged, 

 says Newport, " in a slightly twisted or spiral direction along the 

 margin of the wing, so as to resemble a screw, and when the wings 

 are expanded attach themselves to a little fold on the posterior mar- 



