INTERNAL ANATOMY OF INSECTS. 185 
anal organs have their appropriate motions. In nume- 
rous Coleoptera, however, and some Hemiptera, the up- 
per-side of the abdomen is almost the only part that is 
moveable, especially near the trunk; the wnder-side, hav- 
ing its first segments soldered together, is only capable 
of motion near the tail?. The muscles that produce the 
various motions of this part must be entitled to all the 
denominations stated above”. I have on a former oc- 
casion explained to you how, in insects that have a pe- 
tiolate abdomen, that part is elevated and depressed‘. 
In those with a sessile one the base is attached to the 
metaphragm by strong ligaments‘, and the muscles that 
move the first piece act from one segment to another. 
The partial movements of the segments of this part, 
where they have place, are produced by muscular fibres 
which extend from the whole anterior margin of one to 
the whole posterior one of that which precedes it. If 
those, for example, of the back contract, the abdomen be-~ 
coming shorter above, bends upwards; and if those of 
the sides or belly, it bends sideways or downwards ¢: this 
is a beautiful as well as simple contrivance. 
The alternate rush of air from the abdomen into the 
alitrunk, and from the atmosphere into the abdomen, is 
attended by the constriction or expansion of that part as 
it rises or falls in flight‘, which seems to require the ac- 
tion of constrictor and laxator muscles. : 
iv. The Viscera. Having before had occasion suffi- 
4 Chabrier Sur le Vol des Ins. c.i. Addend. 298. 
> See above, p. 171—. * Vor. LIT. p. 701—. 
4 Chabr. ubi supr. c. 1. 422. © Cuv. Anat. Comp. i. 451. 
* Chabr. Analyse 25. Sur le Vol des Ins. c. 1. 423, 452.  Ad- 
dend. 301, 
