878 



INSECTA. 



Fig. 366. 



Section of pupa of sphinx ligustri ; 1 to 13, dorsal 

 surface, number of segments ; 1 to 12, ventral sur- 

 face, number and position of ganglia ; a, dorsal ves- 

 sel; b, its lateral muscles; c d, oesophagus and sto- 

 mach ; e, ilium ; f, hepatic vessels ; g, colon ; h, 

 rectum; i, double testis; k, brain. Newport, Phil. 

 Trans. 



and form the divisions between the legs of the 

 perfect insect, the bony processes of the 

 sternal surface to which some of the principal 

 muscles are attached. On the upper surface 

 of the same segments they in like manner be- 

 come the phragmata, or bony partitions of the 

 dorsal surface. The fifth segment becomes al- 

 most entirely atrophied, and the sixth very 

 much shortened. A part of the fifth segment 

 forms a portion of the posterior surface of the 

 thorax of the perfect insect, (Jig. 366) while the 

 remainder constitutes the petiole or neck which 

 connects the abdomen with the thorax, the 

 sixth being the first true segment of the ab- 

 dominal region. Exactly the same changes 

 take place in Hymenopterous insects, and in 

 every other species in which we have had 

 opportunities of watching them. We have 

 before alluded to the opinion of Dr. Ratzeburg 

 that the head in Hymenopterous insects is com- 

 posed of two segments of the larva, because 

 just before the change into the nymph or pupa 

 state a portion of the head is found beneath the 

 integuments of the second segment. The fact 

 is indisputable, but the explanation of it appears 

 to be this. The true head of the Hymenopterous 

 larva consists of but one segment, which is 

 provided with the organs of manducation and 

 sensation the same as in the Lepidopterous. 

 But the head in this larva ceases to become 

 larger after a certain period, while the other 

 segments of the body continue to grow, and 

 ultimately acquire a diameter more than double 

 that of the head. Now the parts which are 

 to form the head of the future nymph continue 



Section of perfect state, Sphinx ligustri ; letters and 

 figures as in section of pupa. Newport, Phil. Trans. 



also to grow beneath the unyielding cranium, 

 from which, as the change approaches, they 

 become detached, and are gradually developed 

 backwards, and encroach upon the anterior 

 portion of the second segment. This, in ac- 

 cordance with the laws of development, as 

 established by Geoffrey St. Hilaire, that in 

 proportion as one part of an organized body is 

 increased beyond its ordinary size, the part 

 or parts in its immediate vicinity are in a cor- 

 responding degree arrested in their develop- 

 ment, becomes so much reduced, that in the 

 nymph, this second segment, which in the 

 larva is of the same size as the third and suc- 

 ceeding ones, has not half its original extent, 

 and being still further reduced in that state con- 

 stitutes at length the atrophied, and almost ob- 

 literated pro-thorax of the perfect insect. But 

 while the second segment is thus encroached 

 upon by the first it is in like manner encroached 

 upon from behind by the third, the immense 

 meso-thorax, which supports the chief organs 

 of flight in the perfect insect. The fourth 

 segment from the same cause is developed 

 backwards, and the fifth, diminished to a very 

 small size, exists only as in the sphinx as the 

 petiole which connects the thorax with the 

 abdomen, thus leaving the nine posterior seg- 

 ments of the larva to the latter region, as stated 

 when alluding more particularly to the number 

 of segments in hymenopterous larvae. The 

 necessity for this additional segment in the 

 abdomen of these larvae is a matter of much 

 interest, and appears to be connected with the 

 development of an apparently additional organ 



