INSECTA. 311 



N 



mach ; (see Catalogue of the PJiysiol. Series ofComp. Anat. contained 

 in the Museum of the Roy. Coll. of Burgeons, I. 1833, pp. 189, 190). 



The Diptera live long in the larva-state, but ordinarily very 

 briefly as perfect insects (flies however live long in. this last state). 

 Their larvas have no feet, but some of them have appendages which 

 resemble them, or small hooks, which serve for motion or holding 

 fast, as for instance, the larva of (Estrus. All these Insects undergo 

 complete metamorphosis. Some Iarva3 cast their skin before chang- 

 ing into pupge, and some in addition spin themselves up. Others, 

 on the contrary, do not cast their skin, but this shrinks, hardens, 

 and forms for the pupa, that resembles an egg, a kind of shell or 

 case (pupa coarctata, see above, p. 273). The internal parts separate 

 themselves from this shell, and the change into pupa occurs within 

 this integument, which at last is deserted by the perfect insect 

 when it breaks off the uppermost part in the form of a lid. 



Many of these animals are injurious to us by their puncture ; 

 others suck the blood of our domestic animals j some spoil our food 

 by depositing their eggs on it, especially on flesh and cheese, where 

 the larva (maggots) are developed. There is, on the other hand, no 

 single species of this order from which we immediately derive 

 advantage. Yet so much the greater is the utility they afford 

 us indirectly. Some limit the number of injurious caterpillars, 

 in which they lay their eggs, and which are fed on by the pupge. 

 Others free the air from pestilential exhalations by feeding on 

 carrion and putrescent matters 1 . 



Family IX. Pupiparce. Haustellum of three unequal setee, 

 exsertile from an aperture at the lower part of the head; at the 

 sides of the retractile haustellum two laminse, inarticulate, pilose, 

 porrect. Antennas very short, biarticulate, or with a single pilose 

 joint. Head received behind in the emarginate thorax, or re- 

 sembling a tubercle set upon the thorax. Feet short, strong, 

 remote, furnished with two incurved claws. Wings divaricate, 

 sometimes very short ; in some, together with the poisers, entirely 

 wanting. Body depressed, covered with a hard and elastic skin. 



Pupiparous insects suck the blood of mammals and birds. The 

 buccal organs pass as fine threads through a small opening (just like 



1 It is however somewhat hyperbolical, when Linnaeus says of Musca vomitoria : 

 " Tres muscce consumunt cadaver equi, ceque cito ac leo." Syst. Natur. Ed. xn. i. p. 990. 



