OUTLINES OF ENTOMOLOGY. 91 



from which exudes a sticky fluid, by means of which the insect is en- 

 abled to walk up and down the panes of a window or along the ceiling 

 of a room. The old, ingenious theory of "the exhaustion of air under 

 its feet," by which the crawling of a fly on such surfaces used to be 

 explained, is now known to be erroneous. In the abdomen of the 

 stouter-bodied flies it is difficult to distinguish more than four seg- 

 ments, the terminal ones being abruptly narrowed and drawn within 

 the body to form the ovipositor. 



Such flies as the house-fly and the gad-fly are on the wing and 

 troublesome only during the day. Others, like the mosquito, are most 

 active at night, while some are equally tormenting during the entire 

 twenty- four hours. 



The transformations of dipterous insects are complete. The eggs 

 are deposited singly or in masses upon the solids or fluids upon which 

 the larvae feed. Those of many species are smooth and white and of 

 a linear oblong shape. 



The larvae of terrestrial flies are called maggots. They are soft, 

 thin-skinned, cylindrical, and taper most toward the head, or rather 

 the mouth, for but few of them have any distinct head. They have no 

 legs or other organs of locomotion, and wriggle from place to place by 

 a peculiar twisting of the body, or, as in the case of the " cheese skip- 

 pers," they coil themselves up and seize the tail between the jaws, and 

 then by suddenly letting go, jerk themselves to great distances by the 

 rebound. 



Aquatic larvae are furnished with fin-like swimming organs, and 

 some species breathe through long tubes situated on the posterior end 

 of the body, which can be elevated above the surface of the water. 

 Many dipterous larvre are parasitic ; others feed upon decaying animal 

 or vegetable matter ; those which are aquatic subsist on organic im- 

 purities of water, and a considerable proportion feed on the tissues of 

 growing plants. Except in the case of some aquatic species, the pupae 

 are inactive. They are of two forms : coarctate, that is, inclosed in the 

 dry and hardened larva skin, or obtected, with the larva skin thrown 

 off, and the rudimentary members of the mature insect separately en- 

 cased, as in the pupae of Hymenoptera and Coleoptera. The pupa 

 state is generally of short duration. The Diptera may be considered 

 under two sub-orders : 



I. O R, T H o R A P H A, in which the obtected pupa escapes from 

 the larval skin through a cross slit or T-shaped opening between the 

 seventh and eighth joints; and 



II. y c L o R A P H A, including mostly coarctate pupae, from 

 which the perfect fly escapes through a circular hole on top of the 

 puparium. 



