340 FOREST ENTOMOLOGY. 



tion of mouth - parts. Thus the house - fly takes its food by a 

 sucker arrangement ; and the equivalent of mandibles and maxillae, 

 &c, of other insects are modified into organs for piercing in the 

 mosquito. The antennas of the order vary much in length and 

 structure. 



The legs are usually very slender, and vary much in size. The 

 legs of the house-fly and " daddy-long-legs " are cases in point. The 

 feet end in two claws and a sort of pad. The wings are also variable, 

 and the venation requires the greatest attention, so far as detailed 

 studies are concerned. 



The abdomen is generally made up of eight segments, indicated by 

 numbering from the base that is, the part attached to the thorax. The 

 last or anal segment is of peculiar shape, and contains the generative 

 appendages, and consequently the sexes are very easily determined in 

 many tribes of the Diptera. These characters are, of course, variable, 

 but in the male they may be either a sort of double forceps with 

 central penis, or lamella? and penis sheath. In the female the last 

 segment is prolonged into an ovipositor or vaginal sheath, and has 

 two basal lobes. 1 



With regard to classification, the order may be divided into two 

 sections (1) Orthorrhapha and (2) Cyclorrhapha. 



(1) The larva in this section has a more or less perfectly developed 

 chitinous head, and the pupa escapes by a T-shaped rent in the larval 

 skin. 



(2) In this section the larva has no chitinous first head, but is 

 quite acephalous, and the pupa escapes from the larval skin by a 

 circular opening. 



The Orthorrhapha are divided into two sub-sections viz., the 

 Nematocera and the Brachycera, the former having threadlike and 

 many-jointed antennae, the latter having the antenna? composed of 

 two or three segments with a lateral bristle or arista. 



The principal families of Diptera which cause injuries in the forest 

 are Cecidomyiidce, Phytomyzidce, and Agromyzidce. 



The first-named is numerically considered the strongest, and the 

 following characters of the family are given by Mr Theobald : 2 



1 The student who wishes to pursue the special structural details of Diptera 

 would do well to consult a very detailed chart by the Rev. W. J. Wingate, in the 

 'Transactions of Nat. Hist. Soc. of Northumberland and Durham,' vol. ii., 1906. 



2 An Account of British Flies, by Fred. V. Theobald, pp. 50, 51. 1892. 



