INSECTA. 



921 



rior portion of which, according to Mr. Ma- 

 cleay, is the proper scutellum (5) enormously 

 enlarged, while Audouin regards it as being 

 the dorsal surface of the fifth segment of the 

 larva, so that, if the latter opinion be correct, 

 the thorax of Hymenoptera must be composed 

 of four instead of three segments. We must 

 confess that at first we were inclined to Au- 

 douin's opinion, more especially on account of 

 what we shall presently find in Lepidoptera, 

 in which the fifth segment, in its atrophied con- 

 dition, is as much connected with the thorax as 

 with the abdomen. On further examination, 

 however, we are satisfied that that portion of the 

 meta-thorax which is posterior to the incisure 

 belongs to the third segment of the thorax ; but 

 we differ from Macleay in regarding it rather 

 as the scutellum and post-scutel!um united, 

 than as the scutellum alone. Its proper boun- 

 dary is marked on each side of the segment by 

 an elevated ridge or fraenum, (4 d,} which is 

 extended across the incisure from a little behind 

 the insertion of the wings, where it is conti- 

 nuous with a ridge of the meta-notum, as far 

 as the posterior margin of the acetabulum for 

 the insertion of the coxa of the leg. The post- 

 scutellum, therefore, may be regarded as having 

 coalesced with the scutellum, and assisted in the 

 enlargement of that part. It is distinct, but of 

 small size in Polistes, and is connected at its 

 upper part with a short ligament, or J'uniculus, 

 that is attached to the anterior margin of the 

 sixth segment (6), the first segment of the ab- 

 domen, which it assists to support. But we 

 have yet to trace the fifth segment of the larva, 

 which at first appears to be entirely lost. On 

 carefully separating or removing the meta-tho- 

 racic coxae of Ichneumon Atrvpos, we find a 

 very short plate, reduced almost to a ligament, 

 but still distinct as the remains of a separate 

 segment. It is the connecting medium be- 

 tween the under surface of the thorax and ab- 

 domen. We regard it as the remains of the 

 ventral plate of the fifth segment, of which the 

 upper or dorsal plate has entirely disappeared, 

 or exists perhaps in an altered form, as the fu- 

 niculus just alluded to. We are strengthened 

 in this opinion by an examination of several 

 species of Ichnettmonuhe, although in the gene- 

 rality of Hymenoptera the fifth segment ap- 

 pears to have coalesced with the sixth, to form 

 the petiole or peduncle of the abdomen. The 

 meta-sternum is formed of the same parts as in 

 the preceding segments. The paraptera are 

 situated immediately beneath the posterior 

 wings, in the triangular space bounded in 

 front by the epimeron of the preceding seg- 

 ment, and above and behind by the incisure 

 and frsenum, ( 4 d,) that connect the scutellum 

 with the scutum. The episternum is concealed 

 by the preceding segments, and the sternum is 

 reduced to a small triangular piece, situated be- 

 tween the coxae. The epimeron (4 h) is large, 

 to give attachment to the large coxae, but the 

 trochanlin does not exist as a piece distinct 

 from the coxa (/), with which it appears to 

 have become united. The meta-thoracic or 

 second pair of spiracles (**') are situated in the 

 anterior lateral parts of the scutellum. The 



situation of the spiracles has sometimes been 

 considered as indicatory of the different seg- 

 ments, but, as remarked by Mr. Macleay, these 

 parts are unsafe guides, since they exist in 

 certain segments in some species, but not in 

 others, and their situation is often changed 

 during the metamorphoses from the larva to 

 the perfect state. We have seen that the meta- 

 thoracic spiracles of the larva are placed at the 

 most posterior part of the fourth segment, (fg. 

 356,) but in the perfect insect, as we now find, 

 this is not the case. If the situation of these 

 parts were alike in the two states of the insect, 

 there would be no difficulty in identifying the 

 segments of the imago with those of the larva. 

 W r e believe, however, that the true thorax is 

 formed of the second, third, and fourth seg- 

 ments in all insects, and that the fifth segment, 

 always greatly reduced in size, and sometimes, 

 as in this order, almost entirely atrophied, is 

 not in reality a part of the true thorax, but is 

 sometimes connected more or less with that 

 region, or with the abdomen, being intermediate 

 between the two. Hence we have ventured to 

 designate it the thoracic o-abdominal segment. 



The number of segments in the abdomen of 

 perfect Hymenoptera appears on a cursory ex- 

 amination to vary considerably ; those in which 

 the abdomen is supported on a pedicle or foot- 

 stalk having fewer than others in which the 

 abdomen is of the same width as the thorax, 

 and the sting or borer of the female is not con- 

 cealed, as in Sirex juvencus. This insect on a 

 cursory inspection seems to have nine segments 

 in the abdomen, besides a very large terminal 

 joint, more than twice as large as any of the 

 others, which is pointed at its extremity, and 

 on the under surface of which is situated the 

 anal aperture. In reality, however, there are 

 but nine segments in this most developed form, 

 of abdomen, the tenth being only a large meta- 

 thoracic post-scutellum, which is extended 

 over the base of the abdomen, while the thir- 

 teenth and fourteenth segments of the larva., 

 instead of becoming atrophied, as is usually 

 the case in other insects, during the metamor- 

 phoses, have coalesced and become enormously 

 enlarged in order to afford sufficient space for 

 the muscles required for the employment of 

 the strong terebra or borer with which the 

 insect penetrates the solid timber of living trees 

 to deposit her eggs. In the Tentknedtnidte, a 

 in Ailantiis scrophularia , there are nine dis- 

 tinct segments besides the post-scutellum, and 

 this is probably the case in Athulia centifolite, 

 although we can discover but eight distinct 

 ones in that species. W r e suspect that the last 

 three segments in this insect become united to 

 form the parts connected with the female organs. 

 In the males there is the same number of seg- 

 ments as in the females. This is also the case 

 in Ichneumon Atropos (fig. 390), in which there 

 are nine distinct segments to the abdomen 

 besides the minute plate at the base of the 

 sixth, the remains of the thoracico-abdominal 

 segment before noticed. In the wasp, hornet, 

 and bee, only six segments are at first evident 

 in the abdomen, which arises from the circum- 

 stance that the anal segments, which form past 



