OUDERS OF IXSECTS. 



i: 



meiitaiy, 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 



Fi^. 121. — A, The common Flea {Pulex irritaiis) ; B, Larva of the same ; C, Tujia 

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



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

 emerges in about a fortnight more. 



(_)KDKH IX. DU'TER.V. 



The insects of this onler, as implied by its name, have only a 

 single pair of wings — namely, the anterior ])air. The posterior 

 wings are rmlimentary, an<l are represented l)y two clulil)ed fila- 

 ments called "balancers" or " poi.sers " (fig. li!i!). The mouth in 

 the Diptera is suctorial. It consists of a tubular lower lip, enclosing 

 the other p>aits of the mouth, and placed on the unilei' suiface of 

 the head. The antennie are generally small and three-jiiinteil, 

 sometimes manj'-joiuted, sometimes feathery. The metamorphosis 

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

 indistinct head, and generally destitute of legs. 



The common Gnat ((Vice pipiens) depi5sits its eggs upion the siu'- 

 face of water, the eggs being cemented t(.igethei' to form a sort of raft, 

 and each having an inferior lid which alhiws the escape of the ]ar\'a 

 (fig. 12:2, A). The larval Gnat (fig. 122, B) is vermiform, with a 

 large head and tlKjrax, and with latei-al tufts of bristles, which it 

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

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

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

 long tube, into wdiich the trachea; open, and which the lai'^'a thrusts 

 above the surface of the water for the ])urpose of obtaining air. In 

 its pupal condition (fig. 122, 0) this abdominal tube lias disappeared, 

 though the abdomen still acts as a swimuiing-orgaii. The head and 

 thorax are now fuseil together, and the dorsal asjiect of the tlujrax 

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



