48 2 Studies in the Theory of Descent. 



known. Two distinct larval types here occur, 

 the one caterpillar-like possessing a distinct 

 horny head provided with the typical masticatory 

 organs of insects, and a body having thirteen seg- 

 ments, to which, in addition to a variable number 

 of abdominal legs, there are always attached three 

 pairs of horny thoracic legs : the other type is 

 maggot-shaped, without the horny head, and is 

 entirely destitute of mouth-organs, or at least of 

 the three pairs of typical insect jaws, and is also 

 without abdominal and thoracic legs. The number 

 of segments is extremely variable ; the larvae of 

 the saw-flies have thirteen besides the head, the 

 maggot-shaped larvae of bees possess fourteen 

 segments altogether, and the gall-flies and ichneu- 

 mons only twelve or ten. We should be much 

 mistaken also if we expected to find connecting 

 characters in the internal organs. The intestine 

 is quite different in the two types of larvae, the 

 posterior opening being absent in the maggot-like 

 grubs ; at most only the tracheal and nervous 

 systems show a certain agreement, but this is not 

 complete. 



The order Hymenoptera, precisely speaking and 

 conceived only morphologically, exists therefore 

 but in the imagines ; in the larvae there exist only 

 the caterpillar- and maggot-formed groups. The 

 former shows a great resemblance to Lepidop- 

 terous larvae, and in the absence of all knowledge of 

 tho further development it might be attempted to 



