HALF nOURS WITH ESTSEGTS. 



[Packakd. 



(Fig. 5) bearing a close resemblanco to the common house- 

 fly, are devoted to this work of parasitism-extraordinary. 

 The process is thus : the parent fly lays its eggs within or 

 upon the body of its victim, most commonly a caterpillar 

 which is full-grown and about to enter upon its transforma- 

 tions. The young worm hatches and feeds upon the large 

 stores of fatty tissues which surround the vital organs of its 

 host. How very nice must be the adjustment of relations 

 between the two animals, when in the case of the parasite, 

 the slightest deviation from its path involving any injury 

 to the neighboring nerves or vessels of its host will bring 



Fig. 5. 



Tachina and larva. 



eventual ruin to itself b}^ hastening the death of the cater- 

 pillar ! This delicate balance between the growing parasite 

 and wasting victim may continue through the chrj'salid 

 state of both insects, until just as we think the butterfly 

 will break its prison walls, lo ! the trim, neat form of its 

 unrelenting enemy steps forth from its body ready armed 

 and equipped for active service. 



The late Mr. Ilerrick some years ago announced the dis- 

 covery of the fact that the eggs of the canker worm were 

 preyed upon by a minute ichneumon-fly. Here is an anom- 

 aly — an egg nourishing its inhabitant and immersed in the 



16 



