22 



THE CLASS OF INSECTS. 



antennae, as in the instance of Mantis tessettata described by 

 Lacaze-Duthiers (Fig. 23). In the Cockroach these append- 

 ages, sometimes called "anal cerci," resemble the antenna? of 

 the same insect. In the Lepidoptera and Itymenoptera they 

 do not appear to be jointed, and are greatly aborted. 



The Wings. The wings of insects first appear as little soft 

 vascular sacs permeated by tracheae. They .grow out in the 

 preparatory stages (Fig. 27) of the pupa from the side of the 

 thorax and above the insertion of the 

 legs, i.e. between the epimerum and 

 tergum. During the pupa state they 

 are pad-like, but when the pupa skin is 

 thrown off they expand with air, and 

 in a few minutes, as in the Butterfly, 

 enlarge to many times their original 

 size. The wings of insects, then, are 

 simple expansions of the crust, spread 

 over a framework of horny tubes. 

 These tubes are really double, consist- 

 ing of a central trachea, or air tube, 

 inclosed within a larger tube filled with 

 blood, and which performs the functions of the veins. Hence 

 the aeration of the blood is carried on in the wings, and thus 

 they serve the double purpose of lungs and organs of flight. 



The number and situation of these veins and their branches 

 (veinlets) are of great use in separating genera and species. 

 The typical number of primary veins is five. They diverge 

 outward at a slight angle from the insertion of the wing, and 

 are soon divided into veinlets, from which cross veins are 

 thrown out connecting with others to form a net-work of veins 

 and veinlets, called the venation of the wing (Figs. 28, 29). 

 The interspaces between the veins and veinlets are called cells. 

 At a casual glance the venation seems very irregular, but in 

 many insects is simple enough to enable us to trace and name 

 the veinlets. The five main veins, most usually present, are 



FIG. 27. The semipupa of Bombus, the larva skin having been removed, show- 

 ing the two pairs of rudimentary wings growing out from the mesothorax () and 

 metathorax (m). n and the seven succeeding dots represent the eight abdominal 

 stigmata, the first one (n) being in the pupa situated on the thorax, since the first 

 ring of the abdomen is in this stage joined to the thorax. Original. 



Fig. 27. 



