- ANATOMY AND PHYSIOLOGY. 145 
tions necessarily imply the existence of voluntary 
control over the admission and expulsion of air, and 
we have seen how it is provided for in the structure 
of the spiracles. 
The organs of the circulation are altogether much 
more difficult to trace than any parts which have yet 
come under notice. None but a practised insect 
anatomist can hope to do more than obtain a view of 
some portions of the main arterial trunk, and not 
even this much can be effected without great pains 
and patience. 
The movements of the wasp’s circulation may be 
most easily watched through the transparent skin of 
the larva, by placing it in the live-box, with just pres- 
sure enough to steady it and to bring its skin into 
contact with the glass over a sufficiently large surface. 
The motions languish and cease if the larva is im- 
mersed in water. The actions thus observed, how- 
ever, convey the impression of a very imperfect 
_eirculation, of mere motion rather than of motion in 
one uniform direction. For there seems to be nothing 
more than an imperfect peristaltic closure of a mem- 
branous tube. 
We might naturally have expected to find the 
mechanical arrangements more complete in the per- 
fect insect than in the larva. But Carus* assures us 
that it is not so. The heart, the central pulsatmg 
vessel, remains almost unchanged through all the 
metamorphoses; and the smaller ramifications of the 
blood-vessels, instead of being multiplied and de- 
* Carus. ‘ Traité Elémentaire d’Anatomie Comparée.’ Traduit 
par Jourdan. Tome II, p. 323. 
L 
