1G4 



HINTS ON TIIK ANCfl!STRY OF INSECTS. 



lar systems now appear, but the traclieae 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 larvaj 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 



cnga-ied, in Plaiygaster, in building up 

 the rudimentary appendages, while in the 

 flies (Mnscidaj and Corethra) they build 

 up tlie whole body, according to the 

 remarkable discovd^-y 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) iu 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 Ave 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- 

 bhmce to the third and fully developed larval form (Fig. 19G, trj. 



-tg 



196. Third Larva of 

 Polynema. 



* This reminds us (tlioiijili Ganin docs not mention if) of the development of tlie 

 eni'ii-yo of Juhis, tlie Tlioiisiuid leirs, wliicli. accardin>r to Newport, liatclies tliu 

 2.Jl!i day .Tiler llie ejjg is laid. At this period the embryo is partially orfianized. 

 liaving faint traces of sejjments, and is still enveloped in its embryonal membranes 

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

 teen d:iys, wlieu it tlirows oS. its embryoual membrane, uud becomes detached 

 from the shell. 



