ORDERS OF INSECTS. 175 



mentary, being represented by four minute scales placed on the last 

 two segments of the thorax (fig. 121). The larva of the common 

 Flea is a footless grub, which in about twelve days spins a cocoon 



Fig. 121. — A, The common Flea {Pulex irritans) ; B, Larva of the same : C, Pupa 

 of the same. All the figures are greatly magnified, (After Westwood.) 



for itself, and becomes a quiescent pupa, from which the imago 

 emerges in about a fortnight more. 



Order IX. Diptera. 



The insects of this order, as implied 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 fila- 

 ments called " balancers " or " poisers " (fig. 122). The mouth in 

 the Diptera 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 antenna are generally small and three-jointed, 

 sometimes many- join ted, sometimes feathery. The metamorpho.sis 

 is complete, the larva being usually a soft fleshy grub, with an 

 indistinct head, and generally destitute of legs. 



The common Gnat {Culex pipiens) deposits its eggs upon the sur- 

 face of water, the eggs being cemented together to form a sort of raft, 

 and each having an inferior lid which allows the escape of the larva 

 (fig. 122, A). The larval Gnat (fig. 122, B) is vermiform, with a 

 large head and thorax, and with lateral tufts of bristles, which it 

 uses in swimming. The last abdominal segment carries a bunch of 

 leaf-like fins, with which the larva can keep itself suspended, head 

 downwards, in the water ; and the last segment but one carries a 

 long tube, into which the trachete open, and which the larva thrusts 

 above the surface of the water for the purpose of obtaining air. In 

 its pupal condition (fig. 122, C) this abdominal tube has disappeared, 

 though the abdomen still acts as a swimming-organ. The head and 

 thorax are now fused together, and the dorsal aspect of the thorax 

 is furnished with two tubes, which project above water when the 



