THE INSECTA 161 



chambers may often be observed to contract before the pre- 

 ceding wave has reached the aorta. The blood entering the 

 heart through the lateral ostia is driven forward in a steady 

 stream through the aorta. The heart lies in a pericardial 

 sinus formed by a horizontal membrane stretched across the 

 cavity of the body above the alimentary canal. This membrane 

 is perforated by a number of small openings which admit the 

 passage of blood from the perivisceral blood -space to the 

 pericardial sinus. Both the perivisceral space and the peri- 

 cardial sinus of the cockroach are largely filled up by a mass 

 of white lobulated tissue. This is the fat -body, usually 

 abundant in young insects, but more scanty in adults. Micro- 

 scopical examination shows that the young lobules of the fat- 

 body are filled with vacuolated nucleated cells, but in older 

 lobules the cell boundaries break down, the nuclei disappear, 

 and the cavity of the lobule is filled with granules and crystals 

 containing uric acid. The fat-body is clearly a metabolic tissue, 

 but it has no ducts, and it must be assumed that the urates and 

 other waste products of metabolism pass into the blood stream, 

 and are taken up and finally excreted by the Malpighian tubules. 

 There can be little doubt that the feeble development of 

 blood-vessels in insects is correlated with their mode of respira- 

 tion. Instead of the blood being brought to a respiratory 

 chamber like a lung, or a respiratory appendage like a gill, 

 air is brought to all parts of the insect's body by branched 

 respiratory tubes called tracheae. These tubes are readily 

 seen when a cockroach is dissected under water; being full 

 of air, they look like veins of silver. They are involutions of 

 the external integument, are lined by chitin, and the larger 

 trunks and branches are strengthened by a spiral chitinous 

 thickening. The tracheae open to the exterior by apertures 

 called stigmata, of which there are ten pairs, one pair beneath 

 the anterior edges of the mesonotum, one pair similarly situated 

 beneath the metanotum, and eight pairs beneath the anterior 

 corners of the first eight abdominal terga. The stigmata can 

 be opened or closed by means of valves provided with special 

 muscles, the valves of the thoracic stigmata being external, 

 those of the abdominal series internal. The stigmata lead into 

 short, wide tracheal tubes connected together by longitudinal 

 trunks, from which branches are given off and ramify in all 

 parts of the body, the head being supplied with four large trunks 



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