450 DIPTERA CHAP. 



curved hairs. The most important distinction in external form 

 in Dipterous larvae is that while those that are thorough maggots 

 possess no visible head, others have a well-marked one (Fig. 

 225); these are therefore called " eucephalous " : they have a 

 mouth of the mandibulate type. In some other Dipterous larvae 

 the head is more or less reduced in size, and in the acephalous forms 

 there is only a framework of a few chitinous rods to represent 

 it. The nervous system in the most completely headless larvae 

 is very remarkable, all the ganglia being concentrated in a single 

 mass placed in the thorax. The tracheal system exhibits a great 

 variety ; some larvae have stigmata arranged along the sides of 

 the body after the fashion normal in Insect-larvae ; these are 

 called " peripneustic " ; as many as ten pairs of stigmata may be 

 present in these cases, but nine pairs is much more common. 

 Other larvae have a pair of stigmata placed at the termination of 

 the body, and another pair near the anterior extremity, the two 

 pairs communicating by large tracheal trunks extending the 

 length of the body ; these larvae are said to be " amphipneustic " : 

 this is the condition usual in the more completely acephalous 

 larvae. Others have only the terminal pair of spiracles, and are 

 styled " metapneustic." Some begin life in the metapneustic state 

 and afterwards become amphipneustic. In the aquatic larva 

 of Coretlira there are no spiracles, though there is an imperfect 

 tracheal system. Many Dipterous larvae that live in water 

 or in conditions that prevent access of air to the body have 

 remarkable arrangements for keeping the tip of the body in 

 communication with the atmosphere. The stigmata in meta- 

 pneustic and amphipneustic larvae are very remarkable compound 

 structures, exhibiting however great diversity ; their peculiarities 

 and uses are not well understood ; it appears- very doubtful 

 whether some of them have any external opening. Eeference 

 may be made, as to the variety of structure, to Meijere's paper ^ 

 from which we take the accompanying figure of a posterior 

 stigmatic apparatus in Lipara hwens. It appears that there is a 

 compound chamber — " Filzkammer " — terminating externally in 

 lobes ov fingers — " Knospen " and appearing as marks on the 

 outer surface: this chamber is seated on a tracheal tube, and is, 

 Meijere thinks, probably a secondary growth of the trachea 

 coming to the outer surface. It is traversed by what may be 



' Tijdschr. Ent. xxxviii. 1895, pp. 65-100. 



