EXCRETION IN INSECTS 393 



muscles. It is a long tube, usually confined to the ab- 

 domen, and with eight chambers, with paired valvular 

 openings on its sides, through which blood or haemolymph 

 enters from the pericardium. The blood is driven for- 

 wards, the posterior end of the heart being closed, and 

 there is usually an anterior aorta. But, for the most 

 part, the blood circulates in spaces within what is commonly 

 called the body cavity. Such a circulation is often de- 

 scribed as lacunar. The blood may be colourless, yellow, 

 red, or even greenish, and, in some cases, haemoglobin, 

 the characteristic blood pigment of Vertebrates, has been 

 detected. The cells of the blood are amoeboid. Sugars 

 and amino-acids are present in the blood, in which the 

 respiratory function seems to be unimportant in comparison 

 with the distribution of food-materials. 



Body cavity. — It is necessary to distinguish the primitive ccelom 

 from the apparent body cavity of the adult. Sedgwick notes the 

 following characteristics of a true coelom : — It is a cavity which — 

 (i) does not communicate with the vascular system; (2) does com- 

 municate by nephridial pores with the exterior ; (3) has the repro- 

 ductive elements developed on its hning ; (4) develops either as one 

 or more diverticula from the primitive enteron (or gut), or as a space 

 or spaces in the unsegmented or segmented mesoderm. Now, in Arthro- 

 pods the apparent body cavity of the adult is not a true coelom : it 

 consists of a set of secondarily derived vascular spaces ; • it has been 

 called a pseudocode or a haemocoele. The true coelom of Arthropods 

 is very much restricted in the adult. 



The apparent body cavity in which the organs lie, and in which 

 the blood circulates, is well developed in Insects. It includes, intef 

 alia, a peculiar fatty tissue, which seems to be a store of reserve 

 material, which is especially large in young insects before metamorphosis, 

 and is also interesting as one of the seats of " phosphorescence." 



Excretory system. — Although no structures certainly 

 homologous with nephridia have been demonstrated in 

 Insects, the excretory system is well developed. From the 

 hind-gut (proctodaeum), and therefore of ectodermic origin, 

 arise fine tubes, or in some cases solid threads, which 

 extend into the apparent body cavity. Their number 

 varies from two (in some Lepidoptera, for instance) to one 

 hundred and fifty (in the bee). They twine about the 

 organs in the abdominal cavity, and their excretory signifi- 

 cance is inferred from the fact that they contain uric acid. 



Reproductive system. — Among Insects the sexes are 

 always separate and often different in appearance. The 



