10 INTRODUCTION. 



tained a number of lively young of the Mermis albicans. Upon 

 this I placed several of the young caterpillars of the Yponomeuta in 

 order that the worms might gratify their immigrative propensities. 

 I must explicitly remark, that before experimenting with the 

 caterpillars, I carefully examined each with the microscope, in order 

 to ascertain whether it was not already inhabited by young thread- 

 worms. From their softness and transparency, I could ascertain 

 this point with certainty, without in the least injuring them. 

 The event proved that this inspection was necessary, for out of 

 twenty-five individuals which I at first selected, three contained a 

 thread-worm embryo, which was excessively like those in the 

 flower-pots. I published the results of these experiments a year 

 or two back, in an essay upon the thread-worms of insects 1 , from 

 which I quote the following : 



" From amongst those caterpillars which microscopic inspection 

 clearly demonstrated to be free from thread-worms, thirteen were 

 placed in a watch-glass filled with damp earth containing many 

 lively Mermi s- embryos. After eighteen hours I was able to 

 discover Mermis-embryos in five of the caterpillars. On a second 

 occasion, three-and-thirty of the caterpillars of Yponomeuta 

 coynatella, likewise carefully examined and found free from 

 parasites, were in the same way placed in a watch-glass filled 

 with damp earth containing Mermis-embryos. After four-and- 

 twenty hours, fourteen contained Mermis-embryos. Six of these 

 little caterpillars each contained two small worms, whilst in two 

 others there were as many as three worms. I also employed 

 other caterpillars (of three lines in length) of Pontia Crataegi, 

 Liparis chrysorrhoea, and Gastropacha Neustria, which I took out 

 of coccoons where they had passed the winter. They were, in 

 like manner, placed in a watch-glass upon moist earth containing 

 Mermis-embryos. On the next day, among fourteen caterpillars 

 thus treated, I found ten infested with Mermis- embryos ; five of 

 these contained two worms each, and into one even three worms 

 had wandered." It was clear that these young thread-worms 

 had bored their way through the soft skin into the interior of the 

 young caterpillars. 



From the results of the experiments I have just recorded, 

 one must conclude that it is not necessary to turn to the 

 mystical doctrine of equivocal generation for an explanation 



1 See 'Eulomologische Zeitung,' 1850, p. 239. 



