122 INSECTS 



happens to be at the time, and when the parasitic egg 

 is once in the gallery, the mother insect has done her 

 work. The young larva that hatches in the burrow 

 makes its way along until it finds its prey, attaches 

 itself, and calmly enjoys life at the expense of its host. 

 But the mother insect is by no means infallible, and 

 sometimes her ovipositor fails to hit a boring, either 

 because it is too deeply located or because it has been 

 missed by bad judgment; then nothing remains but 

 to try over again. Sometimes, in a vigorously growing 

 tree, the sappy wood grips the ovipositor and holds so 

 tightly that it cannot be moved one way or the other. 

 Every one who has ever tried sawing through a green 

 log knows what sort of grip such wood may have, and 

 the unfortunate Thalessa that is caught in that way 

 is doomed — usually to be picked off by some inquisitive 

 bird, sometimes to die of over-exertion. I have several 

 times tried to draw out ovipositors caught in that way 

 and have never succeeded: the ovipositor always 

 broke under the strain put upon it. 



Maturing as quickly as many of them do, they are 

 able, as a rule, to keep up with any unusual increase 

 of their host, and to enable them to do this even more 

 effectively, some species have developed the remark- 

 able ability of producing a large number of specimens 

 from one egg — polyembryony, as it is called. One of 

 these minute species finds, for instance, a butterfly egg, 

 and in that lays its own egg, so minute in size as not 

 to interfere with the normal development of the cater- 

 pillar which hatches in due time, but with that parasite 

 egg within its body. The caterpillar grows and so does 

 the parasite; but instead of forming a larva and grow- 

 ing normally, this parasitic egg forms a structure which 

 divides and subdivides and gives off segments almost 

 like those of a tape-worm. Each of these segments 



