Insects and their world 



appendages, except in some of the primitive wingless insects of the Sub- 

 class Apterygota. The eighth to eleventh segments are generally smaller 

 than the others, and often hidden beneath the tip of the abdomen; 

 indeed little or no trace is left of the eleventh segment in most insects. 

 This group of segments has appendages, which are modified into the 

 organs of copulation in the male, and of egg-laying in the female. 



There is tremendous variation in these appendages. The more 

 elaborate forms of egg-laying tube, or ovipositor, may, indeed, be long 

 and conspicuous (Fig. 14), and allow the female to lay eggs in some 

 place where they will be protected, or where the larvae will be able to find 

 food easily — to penetrate the tissues of plants, or the crevices of wood, 

 or to pierce the body of another animal. The highly complex male 

 organs are less easily explained. It is not clear why some insects should 

 have evolved such a compUcated mechanism, when others can manage 

 successfully with very simple structures. The structure of the male 

 genitalia has been much studied because in many groups of insects the 

 species can more easily be distinguished by this means than in any other 

 way. Unfortunately the females of the same groups are more difficult to 

 identify. 



Internal Structure of Adult Insects 

 The internal structure of an adult insect is shown diagrammatically in 

 Fig. 16. The intestinal tube (alimentary canal) runs through the body 

 from the mouth to the anus. The food is first drawn into a cavity called 

 the pharynx, which has strong muscles for this purpose, and passes 

 through the oesophagus or gullet, into the crop. In some inseas this is 

 merely a wider part of the main canal, but in others it is a bag-like 

 structure opening to the side, and acting as a reservoir of food. The 

 food is ground up in a gizzard, and then passes into the stomach, or mid- 

 intestine, where nutriment is absorbed from it. At the beginning of the 

 hind-gut, the Malpighian tubules act as kidneys, and discharge uric acid 

 and other nitrogenous waste products to mix with the faeces in the hind- 

 gut. In the rectum, the final section of the intestine, water is recovered 

 from the faeces, which are then discharged as droppings from the anus. 

 The blood-system of insects is not a closed circuit like our own. Most 

 of the general cavity of the body is filled with blood, and the so-called 

 * heart', or aorta is only a tube with valves, which stirs up the blood 

 and gives it a sluggish circulation. The blood of insects carries food 

 materials from the intestine to other parts of the body, and waste 

 products back to the Malpighian tubules. It does not carry oxygen, 

 which is conveyed directly to all the cells of the body by a system of 

 branching tubes called tracheae. The construction and operation of 



30 



