8 INTRODUCTION 



phases in the life of many intestinal worms, I will here recall 

 certain observations of my own on the natural history of the 

 following parasites. 



For a long time the origin of the thread-worm, known as 

 Filaria Insectorum, that lives in the cavity of the bodies of adult 

 and larval insects could not be accounted for. Shut up within 

 the abdominal cavity of caterpillars, grasshoppers, beetles, and 

 other insects, these parasites were supposed to originate by 

 equivocal generation, under the influence of wet weather or from 

 decayed food. Helminthologists were obliged to content them- 

 selves with this explanation, since they were unable to find a 

 better. Those who dissected these thread- worms and submitted 

 them to a careful inspection, could not deny the probability of the 

 view that they arose by equivocal generation, since it was 

 clear that they contained no trace of sexual organs. But on 

 directing my attention to these entozoa, I became aware of the 

 fact that they were not true Filarice at all, but belonged to a 

 peculiar family of thread- worms, embracing the genera Gordius 

 and Mermis. Furthermore, I convinced myself that these parasites 

 wander away when full grown, boring their way from within 

 through any soft place in the body of their host, and creeping out 

 through the opening. How many a butter fly- collector, keep- 

 ing caterpillars for the breeding of fine specimens of butter- 

 flies, must have seen one or more yellowish white thread-worms 

 winding their way out of them ! These parasites do not emigrate 

 because they are uneasy, or because the caterpillar is sickly, but 

 from that same internal necessity which constrains the horse-fly 

 to leave the stomach and intestine of the horse where he has been 

 reared, or which moves the larva of the gad-fly to work its way 

 out of the boils on the skin of oxen. The larvse of both these 

 insects creep forth in order to become chrysalises and thence to 

 proceed to their higher and sexual condition. This desire to 

 emigrate is implanted in very many parasitic insect larvae, and has 

 long been a well-known fact in entomology. Now I have demon- 

 strated, that the perfect, full-grown, but sexless thread-worms of 

 insects are, in like manner, moved by this desire to wander out of 

 their previous homes in order to enter upon a new period in their 

 lives which ends in the development of their sexual organs. It is 

 true that in the boxes and other receptacles, in which one 

 is generally accustomed to keep caterpillars, these creatures 

 perish j they roll themselves together, and from the absence of 



