TEGMINA. 281 



bivalve shell, and upon which I have seen the halteres act, 

 in a manner similar to a drumstick beating on a drum. 

 Kirby and Spence regard them as analogous to the posterior 

 wings ; but they are decidedly parts of, and connected with, 

 the large wings. Somewhat similar organs have been noticed 

 in the large water-beetle, Dyticus marginalisj I have also 

 found them equally developed in the Hydrous jnceus, at the 

 base of the elytra. 



{b) The Tegmina. — This term was proposed by Illiger to 

 designate that variation of wings, in which the substance is 

 intermediate between the true membranous wings and the 

 hard coriaceous elytra. To the substance, of which these 

 kinds of wings are composed, Kirby and Spence gave the 

 name of pergameneous, as somewhat resembling parchment, 

 or vellum ; and the Orthoptera and Homoptera are the only 

 insects which exhibit this kind of wing, or rather wing-cover, 

 since the posterior wings in these orders are membranous. 

 From elytra, the possession of very numerous nervures suffi- 

 ciently distinguishes the tegmina, whilst their thickened sub- 

 stance equally separates them from the membranous wings. 

 Tegmina are also distinguished from elytra by the inner 

 edges, or anal areas folding over each other, instead of meet- 

 ing in a line down the back (see fig. 130, tegmina of a Blatta). 

 Like many (or rather, theoretically, all) membranous wings, 

 tegmina are divisible into three areas, separated from each 

 other by strong longitudinal nerves ; the external or costal, 

 the intermediate, discoidal or apical, and the anal. The po- 

 sition in which these organs are placed in repose, varies ac- 

 cording to the form of the body. Thus in flat insects they 

 are horizontal, but in thick or compressed insects they are 

 deflexed at the sides, sometimes at a considerable angle. We 

 have seen, that in insects having the mesothoracic scutellum 

 very prominent, the anterior organs of flight are more or less 

 horny, and of very little service in locomotion, requiring 



B B 3 



