42 INTERNAL ANATOMY OF INSECTS. 
in some hawkmoths the intermediate ones are set in 
white or pale spots, which gives great life to the animal. 
In general, in perfect insects the most prevalent colour 
is buff, or reddish-yellow. In the larva of the great wa- 
ter-beetle (Dytiscus marginalis) these organs resemble 
the iris of the eye, being circular with concentric rings, 
alternately pale and dark?. 
4. The size of spiracles varies considerably. Those 
in the Jarva last mentioned are so minute as to be scarcely 
visible except under a lens, while those behind the fore- 
legs in Gryllotalpa are a full line in length, and those in 
the pleura of Macropus accentifer, a Brazilian Capricorn 
beetle, are more than twice as long. In the same species 
they are often found of different sizes ;—thus the anal 
pairs in the Dytiscus lately alluded to, I mean in the per- 
fect insect, are much larger than the rest’, probably that 
the animal may imbibe a larger quantity of air when it 
rises to the surface of the water, where it suspends itself 
by the zazl. In those Lamellicorn beetles in which the 
terminal part of the abdomen is not protected by the 
elytra, the covered spiracles are the largest. 
5. Under the next head, the sztwation of spiracles, I 
shall not only consider the part of the body in which 
they are situated, but likewise their positzon in the crust; 
to which last, as it will not detain us long, I shall first 
call your attention. Their position in this respect is 
most commonly oblique ; but in the abdomen of the above 
Dytiscus they are transverse, and in a larva I possess, pro- 
bably of an Elater, they are longitudinal. In spinose 
* Sphine Labrysce Merian Surinam. 34. 
> Pirate XXIX. Fic. 28. A”. 
