INSECTS. 



[ 432 ] 



INSECTS. 



met with in the same situation. Disks of 

 the same kind but larger, and peculiarly- | 

 arranged hairs, sometimes occur upon the ! 

 upper joints of the tarsus (PI. 34. fig. 6, DY- , 

 TISCUS ; and fig. 4 a, APIS). 



The structure of the legs of insects in the j 

 larval state (PI. 34. figs. 32, 33) differs con- 

 siderably from that of the imago as de- j 

 scribed above. 



The wings are dry, membranous, and 

 transparent organs, consisting of two layers, 

 which are confluent at the margins, and are 

 folds of the integument. Between them 

 run canals, commonly called veins, nerves, 

 or nervures, which are more or less numerous 

 and ramified ; and upon their arrangement 

 the distinguishing characters of the genera 

 &c. are sometimes founded (WiNGs). The 

 veins are formed by two wide horny half- 

 canals in the upper and under plates, of 

 which the wings consist. The main veins 

 arise from the point of attachment of the 

 wings to the thorax, and gradually diminish 

 in diameter until they reach the extremity 

 of the wings. The veins convey the circula- 

 ting liquid, and contain each a tracheal 

 branch, which communicates with the 

 tracheae of the thorax. Each nervure con- 

 tains a trachea ; and the blood circulates 

 around it. In flight they are said to be 

 distended and the wings kept expanded, by 

 air from the interior of the body. In some 

 kinds of wings the circulating currents are 

 not confined to narrow channels as in the 

 veins, but traverse a large part of the 

 breadth of the wings (COCCINELLA). 



Most insects have four wings ; but in 

 some the males only are furnished with these 

 appendages. In the Diptera,the posterior pair 

 of wings are rudimentary, being replaced 

 by two little club-shaped bodies, called the 

 halteres, poisers, or balancers. In this order 

 also, and in some insects belonging to other 

 orders, a pair of small and rounded membra- 

 nous or scaly appendages are attached to 

 the back of the case of the first pair of 

 wings, called in the former the squamas 

 halterurn, and in others, alulae or winglets. 

 In some insects, as in the beetles (Coleo- 

 ptera), the anterior pair of wings are hard, 

 horny, and opaque, forming wing-covers or 

 ELYTRA (fig. 362), from the presence of a 

 horny layer ; and the lower wings, which 

 are usually larger, are folded together 

 beneath them, when at rest. In others, the 

 posterior wings disappear, and the elytra I 

 coalesce at their inner margins. Sometimes 

 the anterior wings are homy or leathery at ' 



the base, and membranous towards the 

 summit (fig. 363) ; these are called hem- 

 elytra. At others, all the wings are thin, 

 membranous, and transparent, as in the 

 Hymenvptera and Neuroptera. 



In the Lepidoptera, they are covered with 

 beautiful feathers or SCALES. 



There are also other modifications of the 

 wings of certain insects, adapting them for 

 special functions. In the Orthoptera these 

 modifications are the agents producing (he 

 well-known chirping sounds, as in the male 

 cricket and grasshopper. In the common 

 house-cricket, Acheta domestica, each of the 

 upper wings or elytra exhibits a clear apace 

 near the centre (PI. 34. fig. 10 a), traversed 

 by a single vein only, or at least by a very 

 few veins. This space has received the name 

 of the drum or tympanum. Bounding it ex- 

 ternally is a large dark longitudinal vein, 

 provided with three or four elevated longi- 

 tudinal ridges. Immediately in front of the 

 tympanum, near the base of the elytra, is a 

 transverse horny ridge, tapering outwards 

 and furnished with numerous short trans- 

 verse ridges or teeth, and forming a kind of 

 file or bow (PL 34. fig. 10 V). When the 

 two elytra are rubbed across each other, the 

 bow being drawn across the ridges gives 

 rise to the peculiar sound, the intensity of 

 which is increased by the tympanum acting 

 as a sounding-board. The apparatus of the 

 grasshopper is essentially of the same struc- 

 ture. It must be stated, however, that va- 

 rious other explanations of the origin of the 

 stridulating noise produced by these insects 

 have been given. Thus by some authors the 

 two bows are stated to work across each 

 other, whilst by others the legs are supposed 

 to act against the bow. This subject pos- 

 sesses interest for future observation. 



In other insects, there is a peculiar mecha- 

 nism for uniting the anterior and posterior 

 wings of each side, so that they may be kept 

 steady and may act in unison during flight. 

 In the Lepidoptera, the moths only are pro- 

 vided with a minute hook arising from the 

 base of the costal nerve of the lower wing, 

 and inserted into a socket near the base of 

 the main nerve, on the underside of the 

 upper wing. In the Hymenoptera, there are 

 many such hooks arranged along part of the 

 costal nerve at the anterior and upper 

 margin of the second pair of wings (PI. 34. 

 fig. 13). When the wings are expanded, 

 these attach themselves to a little fold on 

 the posterior margin of the anterior wing 

 (fig. 11 ri), along which they play freely 



