PkyUtu Parallelism in Metamorphic Species. 489 



and structure of the wings, the number and joints 

 of the legs, the peculiar formation of the thorax 

 (fusion of the three segments) ;" and even the struc- 

 ture of the mouth organs varies only within narrow 

 limits. This is in accordance with their mode of 

 life, which is very uniform in its main features : all 

 the true Diptera live in the light, moving chiefly 

 by means of flight, but having also the power of 

 running ; all those which take food in the imago 

 condition feed upon fluids. Their larvae, on the 

 other hand, are formed on two essentially different 

 types, the one which I shall designate as the 

 gnat-type possessing a horny head with eyes, 

 three pairs of jaws, and long or short antenna,, 

 together with a 12- or 13-segmented body, which 

 is never provided with the three typical pairs of 

 thoracic legs, but frequently has the so-called 

 abdominal legs on the first and last segments. 

 The other Dipterous larvae are maggot-shaped 

 and without a horny head, or in fact without any 

 head, since the first segment, the homologue of the 

 head, can in no case be distinguished through its 

 being larger than the others ; it is on the contrary 

 much smaller. The typical insect mouth-parts are 

 entirely absent, being replaced by a variously 

 formed and quite peculiar arrangement of hooks 

 situated on the mouth and capable of protrusion. 



[For recent investigations on the structure of the thorax 

 in Diptera, see a paper by Mr. A. Hammond, in Journ. Linn. 

 Soc., Zoology, vol xv. p. 9. R.M.] 



