ORDERS OF INSECTS. 171 
four minute scales placed on the last two segments of the thorax. 
The larva of the common flea is a footless grub, which in about 
twelve days spins a cocoon for itself, and becomes a quiescent 
pupa, from which the imago emerges in about a fortnight more. 
ORDER VIII. DrpTERA.—The insects of this order, as im- 
plied by its name, have only a single pair of wings—namely, 
the anterior pair. The posterior wings are rudimentary, and 
are represented by two clubbed filaments called “ balancers ” 
or “ poisers” (fig. 86). The mouth in the Dzféera is suctorial. 
It consists of a tubular lower lip, enclosing the other parts of the 
mouth, and placed on the under surface ‘of the head. The 
antennz are generally small and three-jointed, sometimes 
many-jointed, sometimes feathery. The metamorphosis is com- 
plete, the larva being usually a soft fleshy grub, with an indis- 
tinct head, and generally destitute of legs. 
The Dzpéera constitute one of the largest orders of insects ; 
the House-flies and Flesh-flies (AZusca), the Gnats (Cu/ex), the 
Crane-flies (7zpz/a), the Forest-flies (Wzppobosca), and the Gad- 
flies (7abanide) constituting good examples. 
Fig, 87.—Large white Cabbage-butterfly (Pontia brassicae), a Larva or 
caterpiliar ; 6 Pupa or chrysalis ; c Imago or perfect insect. 
; ORDER IX. LEPipopreRa.—This well-known and most 
eautiful of all the orders of insects comprises the Butterflies 
