Insects and their world 



in the Malpighian tubules (Fig. i6). The urine and faeces are mixed in the 

 hind-gut^ and there and in the rectum (the final section of the intestine, 

 which has special rectal glands) water may be absorbed, until only a dry 

 faecal pellet is left. 



The quantity of liquid that is allowed to pass out of the body to waste 

 varies enormously in different insects. Obviously it must be related on 

 the one hand to the amount of liquid in the diet, and on the other to the 

 situation in which the insect lives, and whether or not it is essential to 

 conserve moisture. 



Many caterpillars, for instance, produce a dry 'frass' that is Hke a 

 granular ash. Homoptera, on the other hand, produce a fluid excrement. 

 That of aphids is known as * honey-dew', because it contains a great deal 

 of undigested sugar; aphids apparently swallow sugary plant juices far 

 in excess of their needs, perhaps for the sake of some other substance 

 that is present in minute quantities. Honey-dew is eaten by other 

 insects, notably ants, which keep aphids as a sort of milk-cow. A similar 

 excretion of carbohydrates by coccids on the tamarisk plant has been 

 claimed to be the manna of the Bible. 



A remarkable economy occurs in the fleas, where the larvae eat the 

 droppings of the adults, which contain undigested blood. Flea larvae do 

 not Uve on the host animal, but in cracks and crevices in its nest or lair. 

 This dependence of the larvae on the droppings of the adults may be 

 one reason why fleas are parasites of animals that have a permanent 

 home, and not of nomadic ones. 



Some of the chemical waste products are used in other ways, notably 

 as coloured pigments. Pupae often turn black before the adult emerges, 

 and the melanin pigment is beheved to be a by-product of the chemical 

 processes involved. The pigments that contribute to the pattern of 

 butterflies and other inseas seem to be part of the chemical process, 

 too, but many of the most brilliant colours seen in insects, especially in 

 beetles, are not pigments. They are an iridescence caused by the optical 

 interference that arises when the surface of the body is covered with very 

 fine markings. 



Reproduction 



Insects are bi-sexual, and have distinct male and female individuals; 

 unlike snails or worms, where one individual may be both male and 

 female, either at the same time, or consecutively. 



The normal sequence of events is that the male transfers sperm to 

 the female by means of an intromittent organ, or penis. Within the 

 female the sperm is stored in a spermatheca or receptaculum seminis, so 



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