164 



HINTS ON THE ANCESTRY OF INSECTS. 



lar systems now appear, but the tracheae are not yet formed. 

 It remains in this state for a much longer period than in the 

 previous stage. 



The third larval form only a few live to reach. This is of the 

 usual long, oval form of the larvsB of the ichneumons, and the 

 body has thirteen segments exclusive of the head. The muscular 

 system has greatly developed and the larva is much more lively 

 in its motions than before. The new 

 organs that develop are the air tubes and 

 fat bodies. The "imaginal disks" or rudi- 

 - at mentary portions destined to develop and 

 form the skin of the adult, or imago, arise 

 in the pupa state, which resembles that of 

 other ichneumons. These disks are only 

 engaged, in Platygaster, in building up 

 the rudimentary. appendages, while in the 

 flies (MuscidaB and Corethra) they build 

 up the whole body, according to the 

 remarkable discovery of Weismann. 



Not less interesting is the history of the 

 development of a species of Polynema, 

 another egg-parasite, which lays its eggs 

 (one, seldom two) in the eggs of a small 

 dragon fly, Agrion virgo, which oviposits 

 in the parenchyma of the leaves of water- 

 lilies. The eggs develop as in Platygaster. 

 The earliest stage of the embryo is very 

 remarkable. It leaves the egg when very 

 small and immovable, and with scarcely a 

 trace of organization, being a mere flask-shaped sac of cells.* 

 It remains in this state five or six days. 



In the second stage, or Histriobdella-like form, the larva is, 

 in its general appearance, like the low worm to which Ganin 

 compares it. It may be described as bearing a general resem- 

 blance to the third and fully developed larval form (Fig. 196, ty, 



* This reminds us (though Ganin does not mention it) of the development of the 

 embryo of Julus, the Thousand legs, which, according to Newport, hatches the 

 25th day after the egg ia laid. At this period the embryo is partially organized, 

 having faint traces of segments, and is still enveloped in its embryonal membranes 

 and retains its connection with the shell. In this condition it remains for seven- 

 teen days, when it throws off its embryonal membrane, and becomes detached 

 from the shell. 



196. Third Larva of 

 Polynema. 



