INTERNAL ANATOMY 



747 



which is a vagina. This receives the cement glands and the sebaceous glands, 

 which provide the secretion for gluing the eggs together, and the receptaculum 

 seminis, beneath which is the bursa copulatrix, which is sometimes separated 

 from the vagina. 



Life- History. — This is very varied. In general terms the sperma- 

 tozoa, bound into bundles of sperm at ophores, are introduced by 

 the external copulatory organ, which, when protruded, is surrounded 

 by the claspers in the form of a sheath, into the bursa copulatrix. 

 The ovum, starting from the terminal portion of the ovarian tube, 

 obtains, as it passes downwards, its food-yolk and its shell or mem- 

 brane, the chorion, which has a small gap, the micropyle. On arrival 

 at the junction of the oviduct and the spermatheca, or receptaculum 

 seminis, the spermatozoa enter via the micropyle, and fertilize the 

 egg. These eggs may be strung together, as they are laid, into masses 

 by means of the sebaceous secretion mentioned above. The egg may 

 develop directly into the adult, but this is only in the lowest forms. 

 Usually the egg develops into a larva, which becomes a nymph or 

 pupa, and this into an adult, often called the ' imago.' This series 

 of changes is called a metamorphosis. The pupa may present 

 three forms: 



1. The free pupa, with s^ppenddiges free. 



2. The ohtect pupa, with appendages and body bound together. 



3. The coarctate pupa, in which the pupa is contained in a pupa- 

 rium formed from the larval skin. 



The larva is a most vigorous feeder, but the pupa does not take 

 nourishment as a rule, though there are marked exceptions, espe- 

 cially among the Rhynchota and Siphunculata. 



Habits. — ^We are mainly concerned with the predatory or blood- 

 sucking insects. As a rule, only the female sucks blood, which, 

 apparently, it does with a view of obtaining rich nourishment for 

 its eggs. It is this habit which makes these insects of importance 

 in tropical medicine, because not merely does it cause irritation 

 by the introduction of secretions from the insects' salivary glands 

 and mouth (alimentary canal), but also leads to the introduction 

 into the victim of the germs of disease. Again, as we shall see later, 

 larvae are apt to become parasitic in man and animals, and in this 

 way cause disease. 



Enemies. — Insects and their larvae have many enemies in birds, 

 reptiles, fish, and other insects. Cannibalism is also met with, 

 especially among mosquito larvae. 



Pathogenicity. — The principal disease-spreading insects are the 

 Diptera — for example, the AnophelincB disseminate malaria, the 

 genus Stegomyia yellow fever, and the genus Culex filariasis, and 

 perhaps dengue fever. Some species of Glossina are responsible for 

 the African trypanosomiases, while they and other flies spread the 

 animal trypanosomiases. 



In addition, the common house-fly is an important factor in the 

 dissemination of typhoid and perhaps dysentery in tropical 

 countries. Fleas are now known to be the spreaders of plague, 



