628 CLASS IV. INSECTA. 



rectum. They are wanting in Collembola and aphids. They 

 vary greatly in number from two to a hundred and fifty and 

 an attempt has been made to use their number in the classi- 

 fication of the Insecta.* 



These tubules, as is often the case in glandular structures, are 

 lined by very large cells with very large nuclei, they contain 

 uric acid and urates of soda and of ammonia and also crystals of 

 leucin and taurin. These substances are passed out of the body 

 by the intestine. The products of nitrogenous excretion are 

 also found in certain cells known as pericardial cells, which are 

 found in and about the pericardium, and in the fat-bodies. It is 

 said but it is very doubtful that the fat-bodies store away 

 the urates throughout life (cf. Tunicata) and that the pericardial 

 cells give theirs up to the blood which transmits them to the 

 malpighian tubules. 



The Circulatory System. The body-cavity of an Insect is not 

 coelomic but haemo-coelic. It is full of blood and all the organs 

 of the interior are floating in blood. The blood is usually 

 colourless, but is greenish in those insects which feed on leaves ; 

 sometimes it is yellowish or brownish, and in the " blood-worm," 

 the larva of the Dipteron Chironomus, it is coloured red with 

 haemoglobin. The blood contains leucocytes which arise from 

 certain cellular masses, which differ in position in different 

 insects and closely resemble the fat-bodies. 



The blood is moved about by a pulsating heart, which under- 

 lies the dorsal exoskeleton of the abdomen in the median line 

 and is contained in a pericardium. It consists of a series, usually 

 eight in number, of pyramidal chambers, in whose walls are 

 circular muscle-fibres. The apex of each chamber opens into the 

 base of the chamber just in front and so forms a funnel. On each 

 side of the base of each chamber is an opening or ostium through 

 which the blood enters the heart and is then driven forward. 

 Arriving at the anterior chamber it passes into a non-contractile 

 aorta which traverses the thorax and in the neighbourhood of 

 the brain discharges the blood by an opening into the general 

 body-cavity again. There is also a ventral channel which 

 lies on the nerve-cord and is faintly contractile. It drives the 

 blood from before backward. 



Separating the pericardium from the body-cavity is a perfor- 



* Brauer, SB. Ak. Wien., xci, 1885, p. 237. 



