INSECTA. 



911 



head is found beneath the skin of the second 

 segment, does not depend upon its having be- 

 come confluent or united with a portion of that 

 segment, but upon the development of those 

 parts which already existed in it in the larva, so 

 that the diminution which the second or pro- 

 thoracic segment undergoes is simply an atro- 

 phied condition, which results from the deve- 

 lopment of the adjoining parts, and not from 

 an actual union or coalescence with them ; 

 since in every instance in which apart becomes 

 confluent with an adjoining one, it loses its dis- 

 tinctness of form and character, and does not 

 remain free as when simply atrophied, or ar- 

 rested in its developement. But when the head 

 of the perfect insect is smaller than that of the 

 larva, as in the Lepidoptera, the extent of the 

 pro-thoracic segment is not diminished, unless 

 encroached upon from behind by the enlarge- 

 ment of the meso-thorax, 



The thorax is that region of the body 

 which immediately follows the head, and bears 

 all the organs of locomotion in the perfect 

 insect. It is always composed of three very 

 distinct segments, first, the pro-thorax, which 

 bears the first pair of legs ; second, the meso- 

 thorax, which bears the first pair of wings and 

 second pair of legs ; and third, the meta- 

 thorax, which bears the second pair of wings 

 and third pair of legs. Besides these seg- 

 ments, which are analogous to the second, 

 third, and fourth in the larva state, there is also 

 another, the fifth segment of the larva, which 

 enters in part into the composition of the thorax 

 of the perfect insect, and forms its connexion 

 with the abdominal region. We have already 

 alluded to this in our account of the changes of 

 the larva, (p. 877, 8,) during which we have 

 shewn that at least one segment of the body 

 always becomes atrophied, and very frequently 

 almost disappears, and that this segment is the 

 fifth. But we have not there sufficiently ex- 

 plained that this segment belongs partly to the 

 thoracic and to the abdominal regions, on which 

 account we propose to designate it the tho- 

 racico-abdominal segment, and consequently the 

 number of segments of which the abdomen is 

 composed will depend upon whether or not we 

 include this in that region. For our own parts 

 we prefer to consider it as forming a most dis- 

 tinct part, for reasons which we shall presently 

 explain. Now it has been shewn by M. Au- 

 douin, in an admirable and elaborate series of 

 investigations, that each segment of the thorax 

 is normally composed of four sub-segments, 

 which sub-segments or annuli are each formed 

 of distinct parts, one upper or dorsal, one lower 

 or pectoral, and two lateral. The four annuli 

 thus formed are easily demonstrable on the 

 upper surface of each thoracic segment, but are 

 less readily detected on the pectoral or under 

 surface, in consequence of the parts having 

 there become confluent, in order to afford a 

 greater degree of solidity to the skeleton ; and 

 in consequence also of the diminished extent of 

 the pectoral as compared with the dorsal sur- 

 face, which, as before explained, (page 877,) is 

 dependent upon the greater extent of change 

 that takes place on the pectoral than on the 



dorsal surface during the metamorphoses of the 

 insect. The parts capable of demonstration in 

 each segment, according to the views of Au- 

 douin, are, on the upper or dorsal surface, the 

 pratscutum, scutum, scutellum, and post-scutel- 

 lum ; on the inferior or pectoral surface a single 

 piece, the sternum, and on the lateral two 

 pieces, the episternum and epimeron on each 

 side ; in addition to which there are also two 

 evanescent pieces, which are of considerable 

 size in some species, but scarcely distinguish- 

 able in others. These are the paraptera, por- 

 tions of the thorax not articulating with the 

 sternum, but with the episternum, anterior to 

 each wing, and the trochantin, articulating with 

 the epimeron and coxa of the leg, the parap- 

 tera of the pro-thorax being, according to 

 Audouin, absent. Hence the number of 

 pieces he describes as forming the external 

 thorax are ten for the pro-thorax, twelve for the 

 meso-thorax, and a like number for the meta- 

 thorax, making in all thirty-four pieces. These 

 are parts capable of being demonstrated, if we 

 regard each sternum as formed of two trans- 

 verse pieces united, and corresponding to the 

 episterna and epimera. But as remarked by 

 Mr. Macleay,* each sternum at the maximum 

 of development ought to be regarded, like the 

 dorsal surface of each segment, as composed 

 of four transverse sub-segments united longitu- 

 dinally, and the sides of the same number. If 

 then the four portions on the dorsal surface of 

 each segment, and the sternum on the under, 

 be also divided in the median line, the number 

 of pieces in the thorax will amount to seventy- 

 two. But this number, as Mr. Macleay has 

 well remarked, can never appear together in 

 any insect, owing to the great extent to which 

 some parts are developed, and the consequent 

 atrophy of others. At the same time it must 

 be observed, that if we adopt this, which ap- 

 pears to be the correct theoretical mode of con- 

 sidering the subject, the number of pieces 

 which enter into the composition of the thorax 

 is in reality greater than that given by M. 

 Audouin, who has not described any parts be- 

 longing to the pro-thorax as analogues of the 

 paraptera of the meso- and meta-thorax, but 

 which we think may be found in a pair of those 

 little detached plates that exist in the articu- 

 lating membrane between the head and pro- 

 thorax in Coleoptera, and which have been 

 described by Straus Durckheimf as pieces 

 jugulaires, and conceived by him to represent 

 the remains of two distinct segments, situated 

 originally between the head and pro-thorax, but 

 which have disappeared during the transforma- 

 tions. But we are more inclined to consider 

 them as detached portions of the pro-thorax 

 than as remains of distinct segments, since we 

 are totally unaware that any such disappear- 

 ance of segments ever takes place between the 

 head and pro-thorax ; the head or first segment 

 of every Coleopterous larva being the proper 

 representative of the head of the perfect insect ; 

 and the second segment of the larva being in 



* Zoological Journ. vol. i. p. 177. 

 t Considerations Gen. &c. p. 75. 



