TRACHEATA 371 



jointed, and terminates in two claws ; between these is the 

 pulvUlus, a fleshy vesicle with tubular hairs, which secrete a 

 sticky fluid. 



The abdomen may be short and conical, or long and 

 cylindrical. There is no true ovipositor, but the terminal 

 segments are retracted into the preceding, and can be pro- 

 truded telescopically. 



The nervous system is very concentrated ; in some families, 

 as the MtrsciDAE, the Hippoboscidae, and the Oestridae, there 

 is, besides the brain, only one large ganglion ; this is situated 

 in the thorax, and gives off nerves to the abdomen. 



The alimentary canal has a stalked sucking stomach, which 

 opens into the oesophagus. 



The sexes do not show much external difierentiation ; as a 

 rule the eyes in the male are larger, they are sometimes fused 

 together. The eggs are smooth, oval, and slightly curved ; 

 they are often provided with a micropyle. In the Tipulidae, 

 the daddy-long-legs (Fig. 209), they are mature when the 

 pupal skin is cast, and are laid immediately by the imago. 



The larvae are white, fleshy, cylindrical maggots. They 

 may have a distinct head, as in the Tipulidae, or a head may 

 be absent, as in the Muscidae. In the first case they have 

 biting mouth parts and are often parasitic, in the second they 

 suck up liquid nutriment. The ova are usually deposited upon 

 the food, either animal or vegetable, in which the larvae 

 burrow and on which they feed. Many of the larvae are 

 aquatic. They change into pupae within the last larval skin, 

 — pupa coarctata, — or cast this skin and become moving pupae 

 which swim in water — pupa dbtecta. 



Sub-order 1. Aphaniptera. 



The Pulicidae. — This is an aberrant order of Diptera ; 

 it comprises the fleas. These insects have no wings, though 

 there are small flat appendages on the meso- and meta-thorax. 

 Their legs are large, and are adapted for leaping. The body is 

 compressed. There is no labrum ; the labium is small, with 

 long palps; the mandibles form serrated stylets, and the 

 maxillae have palps. The female lays eight to ten eggs, which 



