22 THE CLASS OF INSECTS. 



{intenna?, as iii the instance of Mantis tesseUata described by 

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

 ages, sometimes called "•anal cerci," reseml)le the antenna? of 

 the same insect. In the Lepidoptera and Hymenoptera 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 trachea?. The^- grow out in the 



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



jg thorax and above the insertion of the 



/ legs, i.e. between the epimerum and 



qs::ryn — ~\^ /"* tergum. During the pupa state they 



^wf L/^'''>Ak' ^^^ pad-like, but when the pupa skin is 



\ryyj/p'' -4 thrown off they expand with air, and 



L/X/^^'r^ in a few minutes, as in the Butterfly, 

 11 \t^l^' J enlarge to many times their original 

 /"^-^-^ ^ size. The wings of insects, then, are 

 /Cr\\^~^'''''^^ simple expansions of the crust, spread 

 \j\ \^%r^!r over a framework of horny tubes. 



N)^J|iX^ These tubes are really double, consist- 



ing of a central trachea, or air tube, 

 '^' " ' inclosed within a larger tube tilled with 



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

 the aeration of the blood is carrieil 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 live. They diverge 

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

 are soon divided into veinlets, from which cross veins are 

 tlii'own 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 somipiipa oi Bomhus, the larva skin having: been removed, show- 

 ing the tvvo pairs of rudimentary wings growing out IVom the mesothorax (A), anu' 

 metathorax (/»). n and the seven succeeding dots represent the eight abdoininal 

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

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



