72 



NATURAL HIS TOST. 



extremity. These organs are in a constant state of vibration, but what their special function in the 

 economy of the insect may be is unknown. 



The three pairs of legs, which are articulated to the lower part of the thorax, exhibit a con- 

 siderable variety of development, being sometimes of moderate length and stout or even thickened, 

 sometimes excessively long and slender, and exhibiting every intermediate form. They are, however, 

 always articulated to the thorax by a conical coxa, have a ring-shaped trochanter, and five-jointed 

 tarsi, of which the first joint is generally much longer than any of the rest : the last joint has at its 

 extremity a pair of claws, and either two or three membranous appendages (pulvilli). 



The abdomen, which may be attached to the thorax either by its whole base or by a slender 

 stalk, consists of from five to eight segments, but generally shows no external peculiarities, except 



the appendages to the generative organs which frequently project from its 

 apex, and are sometimes very complicated in the males. 



Throughout the order the larvae are footless grubs, generally with a 

 soft body, but sometimes leathery, or even nearly horny. Many of them 

 possess a distinctly marked head, which may bear ocelli, but in the 

 majority the head does not differ from the other neighbouring segments, 

 within which it can be retracted. The mouth is frequently provided with 

 a pair of horny hooks, by means of which the larvae cling to the objects 

 from which they obtain their nourishment. This consists in all cases of 

 fluid materials derived from animal and vegetable substances. The larvae 

 frequently live in the substance upon which they are feeding ; others 

 reside in the water, and many are parasitic. 



When the larva is full grown the change to the pupa state takes 

 place in two different ways. In some the larva skin is cast, and the pupa 

 makes its appearance in a form more or less resembling that of the Lepi- 

 doptera, that is to say, the wings, limbs, antennae, and other parts of the 

 body are shown, only enclosed within a membrane ; while in others the 



DIPTEROUS LARV;E : A MUSCA l arva skin is retained and becomes hardened, so as to form a case within 

 MEDITABUNDA, HEAD NOT which the insect changes into a pupa. The escape of the perfect insect 



B A LE ST"NCT in these latter cases is usuall J effected by the separation of one end of the 

 HEAD. case, like a sort of cap, the agency by which the rupture is effected being, 



ft, stigmata. j n man y ca ses if not in all, a peculiar bladder-like inflation of the forehead 



which is afterwards effaced. Many of the free pupae are provided with sharp hooked 

 processes upon the head and thorax, which are of service to them in making their way 

 out of their places of shelter when about to give birth to the perfect insect ; those of 

 the species living in water are active, and swim vigorously by the action of the 

 abdomen. 



The respiration of the larvae is effected by stigmata, but there is considerable 

 difference in the arrangements of these organs. Many larvae, including the great 

 majority of those provided with a distinct head, have the stigmata, or breathing 

 pores, placed along the sides of the body, two on each segment, as in the majority of 

 the insects which have already been described ; but in a still greater number, in- 

 cluding all those commonly called headless larvae, or in common parlance, maggots, 

 the stigmata are indeed often indicated on the sides of the segments, but these are 

 not perforated, and the only efficient respiratory openings are a pair of stigmata 

 situated at the hinder end of the body, usually upon a flattened surface which COMMON 

 terminates it posteriorly. Some head-bearing larvae, however, which live in water, 

 such as that of the common Gnat, have their sole efficient respiratory aperture 

 situated at the hinder end of the body, where it forms a longish tube, the extremity of which can 

 be brought to the surface of the water for the admission of air to the tracheae, and a somewhat 

 similar provision is met with in some headless larvae. 



In a good many species of the order the larvae are hatched within the body of the mother, and in 

 one whole group, which is distinguished by other peculiarities, they are not only hatched but 



HOUSE 

 FLY EMERGING 

 FROM THE PUPA. 



