86 Objects for the Microscope. 



its divisions into parts or chambers, called cardiac chambers, 

 each of which is closed by a little door or valve, only 

 opening upwards, so that the onward flow of the blood is 

 secured, and out it is forced from the aorta again to con- 

 tinue its circulation. We understand better when we have 

 watched this in the living creature, how the delicate tracheal 

 vessels receiving air through the spiracles give it out to the 

 blood in which they are immersed. Although there are no 

 distinct membranous veins in insects, yet the blood flows 

 in regular channels formed by the interstices of the flakes of 

 fat, air-cells, muscles, &c. The pulsations vary in different 

 insects. Hunter counted thirty-four pulsations in a minute 

 in the heart of a silkworm ; which we can do, as the great 

 dorsal vessel is very distinctly seen constricting and dilating 

 in the full-grown larva. When excited by fear or muscular 

 exertion, the action of the heart is accelerated to as many 

 as 100 and 140 pulsations in a minute. 



In examining the larva of Ephemerae, which are abundant 

 in most ponds, simply confine it in a live box with a drop 

 of water, and just press it sufficiently to keep it still, yet 

 unhurt. If you cannot easily find a larva of Ephemerae, 

 any waterbutt in summer will abound with larvae of Gnat, 

 and they will do nearly as well. So also will a newly- 

 hatched Fly, or a young Bee, just before it emerges from 

 the pupa case ; in these the circulation will be observed in 

 the wings. 



SPIRACLES OP LARVA OF BOT-FLY. 

 ((Brfrw.) 



See egg of (Estrus for an account of the Bot. 



WINGS OF INSECTS. 



There is much to learn in the wings of Coleoptera, 

 Hemiptera, Hymenoptera, and Diptera. The scales or 

 feathers which clothe the wings of Lepidoptera are noticed 

 under Scales of Moth. The Diptera are classed by the 

 veining of the wings, and therefore it is absolutely necessary 

 to have a few specimens mounted as lessons for the names 

 of the nerves or veins of the wing. 



