PARTS or INSECTS. 37 



attention to these parts; later writers have in many instances regarded 

 them more closely; and in the further progress of the science these 

 parts will be consulted with still greater advantage. 



Elytra, or Wing-cases, appertain to the coleopterous order. These 

 are tAvo in number, of a substance rcsemliling leather; for the most 

 part moveable, and opening by a longitudinal suture along tlie middle 

 of the l)ack. These wing-cases or sheaths are often confounded 

 with the wings; but they are really not wings from their structure or 

 substance, nor do they answer the purpose of flight; they merely open 

 to aftbrd the true wing, concealed beneath, the power of expansion and 

 motion, and close down upon the wing when the insect is at rest, to 

 preserve it from injury. Some Colcoptcra have the elytra united. 



The superior surface of the elytra is more or less convex, and the 

 lower surface correspondently concave: the texture in some, as in 

 many of the Ciirculumes and Cenimlij/ces, is so hard that it is pierced 

 with difficulty by means of a strong pin ; in others so flexible that they 

 spring into their proper form immediately after being bent double. 

 The proportions of the elytra compared with the body are various; 

 their form dissimilar; and the diversity of their surface — arising from 

 dots raised or depressed, protuberances, flutings, colours, and other cir- 

 cumstances — endless. These difi:erenccs in the elytra furnish some ex- 

 cellent generic distinctions, and are still more extensively useful in 

 constituting the characters of species. 



H ALTERES, Poisers, or balancers : appendages peculiar to insects of the 

 dipterous order, and which, with sufficient reason, are deemed an essen- 

 tial character of that group. These poisers are t\vo short, moveable, 

 clavated hlaments, placed one contiguous to the origin of each wing. 

 They seldom exceed one-tenth the length of the wing, though in cer- 

 tain genera they are rather longer. The capital, or head, in which the 

 iilament terminates, is either roundish, oval, truncated at tlie end, or 

 compressed at the sides: in some insects its situation is directly under 

 a small, arched, filmy scale, A\hich also varies in size and form; and in 

 several families is apparently wanting. 



The exact purpose to which nature has destined these organs has 

 not been hitherto ascertained in a very satisfactory manner. The 

 most prevalent, and perhaps in some measure the most consistent, 

 opinion seems to be, that they balance or counterpoise with the action 

 of the wings, when the insect is in flight, in tlie same manner as rope- 

 dancers exercise a pole to preserve their equilibrium. The diminutive- 

 ness of their size is a plausible objection to this idea. Others consider 

 these as the organs of that vibratory sound which dipterous insects emit 

 in flight : they compare the filmy scale to a kind of tambour, and liken 

 the balancer to a drum-stick, which striking repeatedly upon it, they 

 conceive, must occasion this noise. It is apprehended the sound they 

 emit in flight cannot be traced to this cause; for the best of all possible 



