from which they pass into the stomach. The larvae attach them- 

 selves to the gastric mucosa, finally passing through the alimentary 

 canal, and assuming the pupa stage outside the body of the host. 



The classification of the insects, especially as regards parasitic 

 and otherwise aberrant forms, is a matter of some difficulty, though 

 some of the common free-living orders are readily recognizable. 

 The features used in classification are principally those of the 

 mouth parts and of the wings, of which there are typically present 

 two pairs attached to the thoracic segments. Orthoptera, in- 

 cluding the grasshoppers and cockroaches, have cutting mouth 

 parts. The anterior wings are tough and leather-like, while the 

 posterior wings are membranous, and when at rest folded. Coleop- 

 tera or beetles have the mouth parts of a cutting type, but the 

 anterior wings are formed into hard protective elytra, which at 

 rest meet in the middle line, though they can be spread apart so 

 that the thin posterior wings may be expanded for flight. Lepi- 

 doptera, moths and butterflies, have suctorial mouth parts, and 

 the wings, often beautifully coloured, are encased in a covering of 

 minute scales. Diptera or flies have suctorial mouth parts, vari- 

 ously modified, the anterior wings well developed and membranous 

 while the posterior ones are vestigal. Hemiptera or bugs have 

 the mouth parts formed into a jointed sucking tube, the anterior 

 wings typically hard at the base and membranous at the tip, 

 though this condition applies more definitely to the so-called 

 Heteroptera, there being some Hemiptera homoptera in which 

 the wings are membranous throughout. Many of the parasitic 

 forms belong to the Diptera and Hemiptera, though highly modi- 

 fied insects, such as the lice, are placed in a separate group of the 

 Ancplura, while others, such as the fleas, are representative of the 

 division of the Siphonaptera, the characters of these forms being 

 described below. 



The common housefly, Musca domestica, an example of the 

 Diptera, owes its prevalence to the common occurrence of exposed 

 waste material of an organic nature in which the eggs are deposited 

 and the larval and pupal stages are passed through. The life 

 history is analogous to that of Pieris, except that the larvae or 

 "maggots" are acephalous, and the pupa is protected by the 

 larval cuticle instead of by a coccon. Though not a biting fly, 

 the mouth parts being suctorial in nature, the housefly is undesir- 

 able on account of its indiscriminate passage from food materials 

 to waste and vice versa, waste materials being liable to bacterial 

 infection, which may be thus communicated to food intended for 

 human consumption. 



The family Muscidae, however, contains many species in which 



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