DIPTERA. 363 



and secrete an adhesive fluid, which is said to aid the fly in 

 walking up-side-down on polished surfaces. 



The nervous system in the Diptera is characterized by a 

 grouping of the thoracic ganglia into a single mass, from which 

 proceed nerves to the abdomen ; the abdominal ganglia being 

 for the most part aborted. Thus in some Muscidce, CEstrus, 

 and Hippobosca, the nervous cord behind the cephalic portion, 

 consists of a single thoracic ganglion, which gives out nerves 

 in different directions. The higher Muscids, such as Syrphus 

 and Conops have in addition one or two ganglia situated at 

 the base of the abdomen. The higher groups, such as the 

 Tabanidce, Asilidce and Bombylidce have six ganglia, 

 and the Empidce, Tipulidce and Culicidce have more. 

 The larvae usually have one more pair than the adult, having 

 ten and sometimes eleven ganglia, with long commissures, 

 which are often double. 



The digestive system is less complex than usual. As in the 

 two preceding suborders, on one side of the oesophagus is a 

 pedicellate sucking stomach which extends into the abdomen 

 near the true chyle-making stomach. The latter is of the 

 usual intestinoid form, enlarging a little anteriorly, with two 

 coecal appendages beneath on each side, near the cardiac ex- 

 tremity. 



The four, rarely five, Malpighian vessels which correspond 

 to the kidneys of vertebrates, are united before they open into 

 the single or double common outlet. 



There are two main tracheae, and two large air-sacs, one on 

 each side, at the base of the abdomen. The system of tracheae 

 is simplest in the aquatic Tipulid larvae, resembling in this 

 respect the Phryganeae, where the tracheae are subcutaneous 

 and designed to extract the air from the water. 



The testes are generally colored, being provided with a pig- 

 ment layer. They are oval, curved or tortuous glands, with 

 a short efferent vessel (vas differens). The ovaries consist of 

 three to four chambered tubes, and a short oviduct. The re- 

 ceptaculum seminis is generally triple. A true bursa copulatrix 

 is wanting in the Diptera, but in "many Muscidce the vagina 

 has, as a seminal receptacle or uterus, a spacious and sometimes 

 two-lobed reservoir in which the fecundated eggs are accumu- 



