INSECT. 



859 



termed by Kirby and Spence the Faniculus, and is 

 serviceable in elevating or depressing the abdomen. 



We will now proceed to the description of the Or- 

 gans of Locomotion ; these are either wings, or their 

 representatives, or legs. 



I. Wings, or the organs of aerial progression. 



These organs, unlike the wings of birds, consist 

 simply of a double membrane, of a very slender and 

 generally transparent consistence, inclosing numerous 

 nervures or veins of a firmer substance. These ner- 

 vures are a kind of solid tubes inclosing the tracheae, 

 or aeriferous vessels in their interior, of which we 

 have already spoken in our account of the escape of 

 the perfect insect from the pupa skin. 



These organs undergo very great modifications of 

 form and structure in the different orders of insects ; 

 their number is also liable to corresponding varia- 

 tions ; we say corresponding, because as one pair of 

 wings is sometimes so completely modified as to be 

 no longer serviceable as an organ of flight, the num- 

 ber of these wings is necessarily reduced from four 

 to two. These modifications occur either in the 

 anterior or posterior pair of wings : thus, in the Cole- 

 optera, the fore-wings, although ample, are trans- 

 formed into a pair of scales serving for the defence of 

 the wings, and unserviceable as instruments of flight. 

 The same occurs in a greater degree in the Strep- 

 siptera. In the Hemiptera (Heteroptera), the four 

 wings are of a leathery structure at the base, but are 

 membranous at the tip ; whilst in the homopterous 

 Hemiptera, and Orthoptera, the upper wings are of a 

 membranous nature, but much thicker than the lower 

 wings throughout. In the Hymenoptera, Neuroptera, 

 and Lepidoptera, all the wings are equally mem- 

 branous. In the Diptera the fore- wings are alone to 

 be found as organs of flight, the posterior wings being 

 reduced to a pair of slender knobbed filaments. 

 There are many cases in which the wings are totally 

 wanting, as in the glow-worm, many Orthoptera and 

 Hemiptera, &c. ; but these are to be accounted only 

 as casual exceptions The same may be said of 

 many beetles, which have the elytra, or upper wings, 

 soldered together ; in which case, as wings would be 

 useless, they are not given to the insect. 



The only recorded instance in which the posterior 

 pair of wings have been ascertained to be developed, 

 without there being the least rudiment of the anterior 

 pair, is found in Perlamurpka hieroglyphica of Curtis, 

 one of the spectre insects (Phasmid<e}; also de- 

 scribed in the last number of the Zoological Journal, 

 under the name of Asckiphasma annulipes. 



The wings arise respectively from the anterior 

 lateral angles of the ineso- and rnetathorax, articu- 

 lating with the scutum and episternum, that is, just 

 at the place where the dorsal plates of the meso-and 

 metanotum unite with the lateral plates of the meso- 

 and metasternum. 



With respect to their consistence, the organs of 

 flight may be divided thus : 



a. Membranaceous wings. 



b. Tegmina. 



c. Hemelytra. 



d. Elytra. 



e. Halteres. 



f. Pseudhalteres. 



a. Membranaceous ivings. It is by means of these 

 membranous appendage, that flight is chiefly effected, 

 the other variations of these organs serving more es- 

 pecially as organs of defence to the true wings. In 



their most simple form, as in some of the minute llymc- 

 'loptera, especially the genus Pailus (fig. 128), the 



128 



wing consists but of the two layers of membrane, 

 w ithout any visible air-tubes, or nerves, which, in the 

 more advanced structure of the wing, are developed, 

 dividing it into more or less numerous divisions, like 

 the frames of a window. This formation is more and 

 more complete, and the divisions more and more nu- 

 merous, until we find the wing exhibiting a net work 

 of meshes too numerous to be counted ; this is the 

 case with the dragon-fly (fig. 129). In the Hymeno- 

 ptera, and Diptera, the wings are essentially similar in 

 the construction ; but the nervures, especially of the 

 posterior wings of the former, are less numerous, 

 forming but few spaces, or cells, as they are termed. 

 The second wings of the Hymenoptera are smaller 

 than the anterior, whilst in the dragon-flies, and some 

 other neuropterous insects, they are of equal size. If, 

 therefore, we regard the development of wings as the 

 chief characteristic of the Ptilota, or winged insects : 

 those species which exhibit the wings of equal consis- 

 tence and size, must be regarded as the types of the 

 tribe. 



The external margin of the posterior wings exhi- 

 bits, also, in many insects with naked wings, minute 

 hooks for retaining the wings on each side together 

 during flight. These are especially found in the Hy- 

 menoptera, and are called hamuli. In the lepido- 

 pterous insects another structure prevails for the like 

 purpose ; the margin of the posterior wing being fur- 

 nished at its base with a long and curved bristle, 

 which is received into a little hook on the under sur- 

 face of the anterior wings, in which it plays. The 

 wings in this same order, offer another peculiarity, 

 since, instead of being naked and transparent, they are 

 clothed with a double layer of minute scales, some- 

 what resembling those with which fishes are covered. 

 These scales, upon which the beauty of lepidopterous 

 insects so entirely depends, are easily detached in the 

 form of a fine dust, which, when examined with the 



Feather- scales from the Goat-moth. 



microscope, are exceedingly variable in their form, 

 but generally more or less wedge-shaped, or oval : 

 sometimes toothed, or notched, at the broadest end, 

 some having a slender footstalk. The membranous 



