924 



INSECTA. 



space, from the sides of which originate the 

 second pair of wings (*). It is bounded pos- 

 teriorly by a short thick ridge, the remains of 

 the scutellum (4 r), the extremities of which 

 pass outwards and are connected with the base 

 of the wings. The post-scute Hum (4 d) is 

 also a very short fold, that forms the most 

 posterior part of the true thorax. It is de- 

 veloped inwards and becomes continuous with 

 the remains of the upper portion of the fifth 

 or thoracico-abdominal segment (5). At each 

 side it is connected with the lateral portions of 

 the scutellum, and with it is connected to the 

 base of the posterior wings, and also with a 

 membrane or fraenum (5*) that passes from 

 the base of the posterior wings to the posterior 

 margin of the thoracico-abdominal segment, 

 thus clearly indicating the relation which this 

 segment bears to the last segment of the thorax. 

 The metaphragma or septum that exists between 

 the thorax and abdomen is formed during the 

 metamorphoses by a constriction in the middle 

 of the fifth segment, and as the changes pro- 

 ceed, a portion of the fourth segment, the post- 

 scutellum, is included in the fold or constriction, 

 and assists to form the metaphragma, so that 

 the fifth segment, at least in Lepidoptera, is 

 common both to the thorax and abdomen, and 

 cannot properly be said to belong more espe- 

 cially to one than to the other. Only a short 

 portion of the fifth segment exists on the dorsal 

 surface of the abdomen, posterior to the thorax, 

 while the inferior portion, which was more 

 reduced in extent during the changes than the 

 upper, is reduced to a very short piece, which 

 has entirely coalesced with the under surface of 

 the sixth segment, the first true segment of the 

 abdomen. In the meta-sternal surface there 

 are the same parts developed as in the meso- 

 sternal, the arrangement of all the parts being 

 precisely similar to those of the meso-thorax. 

 The trochantinus (4 /c) is united with the 

 coxa (/), from which it is distinguished, as in 

 the limbs of the preceding segments, by a 

 lateral suture. The first is articulated ante- 

 riorly with the sternum (4 g), and the second 

 posteriorly with the epimeron (4 h). The 

 second or meta-thoracic spiracle is situated in 

 a deep cavity behind the wings. 



The abdomen in Lepidoptera consists of nine 

 distinct segments, or the remnants of that num- 

 ber of the larva if we include the segment which 

 we have thus seen is connected with the thorax. 

 We prefer, however, to consider the fifth as a 

 distinct segment, although a portion of it covers 

 the base of the abdomen. Each segment is 

 formed, as in other insects, of two arches, a su- 

 perior and an inferior one. The superior one is 

 a strong corneous plate, and is equal to nearly a 

 complete semicircle. The inferior plate is similar 

 in its form, but does not include so large a 

 portion of an arch, and is not so completely 

 solidified. The lateral margins of the inferior 

 arches are nearly straight, but those of the 

 superior ones are emarginated or notched, each 

 notch or incisure being near the middle of the 

 edge. It is occupied by an oval corneous ringf, 

 the stigma or spiracle which exists in the soft 



membrane or conjunctiva that connects the 

 margins of the superior and inferior arches of 

 the segments. A similar membrane connects 

 the different segments together longitudinally 

 in such a manner that the anterior margin of 

 one segment is drawn beneath the posterior of 

 the one that immediately precedes it. By this 

 arrangement of the parts of the segments the 

 abdomen can be elongated or shortened at the 

 will of the insect, and expanded or contracted 

 during respiration, which takes place in the 

 abdominal as well as in the thoracic region. 

 There are nine stigmata or spiracles on each 

 side of the body. Two of these we have seen 

 are situated in the thorax, and the remaining 

 ones in the abdomen, from the sixth to the 

 twelfth segment; but the twelfth is apparently 

 closed, and probably does not take part in the 

 function of respiration, which is carried on 

 chiefly through the thoracic spiracles. It is 

 worthy of note also that there appears to be a 

 change in the situation of one of the spiracles 

 during the transformation of the larva and pupa 

 state. In the larva a spiracle exists in the fifth 

 segment, but this does not seem to be the case 

 in the perfect insect, in which the spiracle is 

 removed forward to the base of the wing in the 

 fourth, a circumstance which is highly interesting 

 from the fact that the wings are directly con- 

 nected with the organs and function of respira- 

 tion. 



We will not enter further upon an examina- 

 tion of the thorax and abdomen in the different 

 orders, sufficient illustrations having been given 

 of the parts of which they are composed, and 

 of the manner in which they are developed 

 from the almost uniform body of the larva. 



3. Organs of locomotion. The wings, the 

 organs of flight in Insects, differ from those of 

 Birds in being supernumerary parts adapted 

 especially for aerial motion, as the legs, the 

 proper organs of progression, are for terres- 

 trial. The wings of Birds are simply mo- 

 difications of the anterior pair of extremi- 

 ties, which are employed in most other Ver- 

 tebrata either as organs of prehension or of 

 terrestrial or aquatic locomotion, and form parts 

 of the normal type of the skeleton.* But 

 the wings of Insects have no more analogy 

 with the legs, the proper organs of locomotion, 

 in the invertebrated than m the vertebrated 

 classes. They are derived entirely from the 

 respiratory structures, and have sometimes been 

 aptly designated aerial gills. They are ex- 

 panded portions of the common tegument of 

 the sides of the meso- and meta -thorax, occa- 

 sioned by the enlargement and extension of 

 numerous trachese and the accompanying pas- 

 sages for the circulatory fluids, and their motions 

 are intimately connected with the function of 

 respiration. These tracheae ramify throughout 

 every part of the wing, and immediately after 

 the assumption by the insect of the imago state 

 become solidified like the rest of the skeleton. 

 They are hollow for the reception of air like 

 the proper respiratory organs within the body, 



* See the Article AVES. 



