INSECTA. 311 
mach ; (see Catalogue of the Physiol. Series of Comp. Anat. contained 
in the Museum of the Roy. Coll. of Surgeons, 1. 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 larve 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 @strus. All these Insects undergo 
complete metamorphosis. Some larve cast their skin before chang- 
ing into pupe, 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 ; some spoil our food. 
by depositing their eggs on it, especially on flesh and cheese, where 
the larve (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 pupe. 
Others free the air from pestilential exhalations by feéding on 
carrion and putrescent matters? 
Family 1X. Pupipare. Haustellum of three unequal sete, 
exsertile from an aperture at the lower part of the head; at the 
sides of the retractile haustellum two lamin, inarticulate, pilose, 
porrect. Antenne 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 Linnzus says of Musca vomitoria : 
“Tres musce consumunt cadaver equi, ceque cito ac leo.” Syst. Natur. Ed. xu. 1. p. ggo. 
