INSECT. 



861 



Dyticus marglnulis ; we have also found them equally 

 developed in the Hydrous piceut, at the base of the 



elytra. 



b. The Tenmina. This term was proposed by 

 Illi<fer to designate that variation of wing-s, in which 

 the substance is intermediate between the true 

 membranous wings and the hard coriaceous elytra. 

 To the substance of which these kiads of wings are 

 composed, Kirby and Speuce gave the name of per- 

 gameneous, 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 sufficiently distinguishes the 

 teguiina, whilst their thickened substance equally se- 

 parates them from the membranous wings. Tegmina 

 arc also distinguished from elytra by the inner edges, 

 or anal are as folding over eacu other, instead of meet- 

 ing in a line down the back (see fig. 130, tegtnina of 

 a Blatia). Like many (or rather theoretically all) 

 membranous wings, tegmina are divisible into three 

 areas, separated from each other by strong longitudi- 

 nal nerves ; the external or costal, the intermediate, 

 discoidal, or apical, and the anal. The position in 

 which these organs are placed in repose, varies ac- 

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

 they are horizontal, but in thick or compressed insects 

 they are deflexed at the sides, sometimes at a consi- 

 derable angle. We have seen, that in insects having 

 the mesothoracic scutellum very prominent, the ante- 

 rior organs of flight are more or less horny, and of 

 very little service in locomotion, requiring some kind 

 of support, which is afforded by the scutellum. They 

 are thus circumstanced in beetles, and in the hetero- 

 pterous Hemiptera ; but in the tribes in which tegmina 

 are present the scutellum is not a prominent organ ; 

 they are therefore more essentially organs of flight 

 than elytra or hemelytra, which we now proceed to 

 describe ; commencing with the former, as being inter- 

 mediate in their formation between tegmina and 

 elytra. 



c. The Hemelytra (see fig. 131, 132, hetnelytron of 

 Jlfffi-offdster urlicce}. Under this name, Latreille 

 described (Hist. Nat. Crust. &c. ii. p. 1(54) the ante- 

 rior organs of flight of the Cimiculce, or heteropterous 

 Hemiptera, the basal portion of which is thick, 

 opaque, and coriaceous ; and the terminal part thin 

 and membranous ; the transition between the two 

 kinds of membrane not being gradual, but sudden. 

 The wind's covered by these limbs are membranous, 

 and of the size of the hemelytra. On examining an 

 hemelytron, the three areas of which wings and teg- 

 mina are composed are here distinct ; but they do not 

 extend beyond the corium (or basal coriaceous part) ; 

 hence the apical membrane seems a piece added to 

 the ordinary structure ; sometimes being, in fact, ab- 

 sent, although the corium is distinct. In some 

 instances, the entire hemelytra are formed of mem- 

 brane, or rather the substance of the corium is so 

 much diminUhed that it resembles the apical mem- 

 brane. When at rest, the apical membrane of one 

 hemelytron folds upon that of the other, and the posi- 

 tion is usually horizontal. As to their neuration, the 



\ corium, us may be conceived from its similarity 

 to tiie su'i-; aii-e of the elytra, is generally almost 

 destitute of nerves ; but the apical membrane differs 

 in having more or less numerous nerves, which serve 

 for the distinctions of genera and sub-genera. 



d. Elytra (see fig. 133). This is the name given 

 to the hard scaly or horny wing covers, or anterior 

 wings of beetles ; they are thickened opaque plates, 

 generally carried, when the insect is not on the wing, 

 in a horizontal position upon the back, with the in- 

 ternal (or anal) margin (or suture, as it is technically 

 termed) straight, and in the direction of the middle 

 line of the body. They are destitute of nerves, and 

 are internally lined with a thin membrane. As to 

 their consistence, they are variable, although gene- 

 rally very hard and horny ; sometimes even so firm 

 as to be with difficulty pierced by a pin. In other 

 instances, however, they are flexible, or elastic, 

 yielding to pressure ; and in others, even soft, vary- 

 ing according to the variations in the consistence of 

 the body. As to their form, they are generally in 

 the figure of an oblong square, having the posterior 

 angles more or less rounded ; at other times they are 

 more triaugular.oval, or even almost semi-hemispheric. 

 Sometimes they entirely cover the abdomen, at others 

 they leave the extremity of this part exposed ; and 

 sometimes, as in the Brachelytra, they are very short. 

 As to their surface, they are flat, convex, gibbose. 

 They are also variable as regards their clothing ; some 

 being quite naked, others hairy, woolly, silky, spiny, 

 squamose, and rugose, like shagreen, or with impres- 

 sed or elevated spots, either placed irregularly (fig. 

 133, o), or disposed in lines (Lineato-punctate, fig. 

 133, j-), or with impressed lines (Elytra striata, fig. 

 133, f). Their colours are very variable, according 

 to the habitation of the insect. Thus beetles, living 

 in dark places, underground, and stones, &c., are 

 generally black, whilst those which are exposed to 

 the light are more variegated in their tints. As al- 

 ready observed, these organs, during flight, are but 

 little serviceable as locomotive instruments; and un- 

 less it were for the manner in which they are carried 

 at such times, it must be evident that, from their size 

 and substance, they would necessarily present a great 

 obstacle against quick motion in the air ; but this is 

 provided against by their being carried either in a 

 horizontal direction, where they are extended later- 

 ally, or in a vertical position, where they do not ex- 

 tend laterally (as in the burying beetles, Necrophagtis). 

 Ordinarily, as their name imports, they cover the 

 wings ; but in some beetles the wings are riot thus 

 defended, owing to the small or irregular form of the 

 wing-covers. In the Bmdielytra, however, in which 

 the elytra are of the least size, and the wings large, 

 they possess their ordinary function, the wings being 

 very much folded. In some beetles the wings are 

 entirely wanting ; and here the elytra are firmly sol- 

 dered together, thus becoming a defence for the ab- 

 domen, the upper surface of which is, in such cases, 

 sofi and membranous. 



e. Haltercs (see fig. 126 and 127, z. z). These 

 organs, which are usually termed balancers or poisers, 

 are a pair of short threads arising behind the base of 

 each of the wings in the Diptera (to which order they 

 are exclusively confined), and terminated by a small 

 oval or triangular knob. These organs are capable of 

 a very rapid motion ; and it has been usually sup- 

 posed that by their beating upon the winglets (alulae), 

 they produce the humming noise so distinctly heard 

 in this group of insects during flight. That they do 

 thus beat upon the winglets, we have already obser- 

 ved ; but it appears doubtful whether this action be 

 the cause of the sound in question, because those 

 Diptera which do not, either by nature or artificially, 



