NEMATOMORPHA LIFE-HISTORY 



171 



terrestrial beetles to the water, but von Linstow suggests that, as 

 they live partly on snails, and at this time of year there are not 

 many land-snails about, they may be in search of water-snails 

 such as Limnaea. They may also be sometimes blown into the 

 water by wind storms, but, whatever the cause is, their pres- 

 ence in water is essential for the continuance of the life of their 

 parasites. 



Once free in the water the Gordius is soon sexually mature ; 

 the fertilisation takes place in April, and 



may be seen twisting and 



the stems of water-plants 



long bead-like strands of 

 The first deposition ob- 

 served by von Linstow took place on 14tli 

 April, the last on 2nd August, and the 

 period of egg -laying for each female ex- 

 tended over four weeks. At first the eggs 

 snow-white, but within twenty- four 



then the female 

 writhing round 

 and laying the 



eggs (Fig. 82). 



are 



Fig 



-The tail ends 

 of a female Gordius 

 (a) and a male (b) in 

 copula, x 1*5. (From 

 G. Meissner. 1 ) 



hours they turn brown in colour. 



The development of the first larva within 

 the egg takes about a month. When it 

 emerges from the egg-shell it is minute, *065 

 mm. long, ringed anteriorly, and provided with a protrusible and 

 retractile boring apparatus consisting of three chitinous rods ; 

 round the base of this piercing proboscis is a double crown of 

 papillae, each bearing a spine (Fig. 90). 



This first larval form breaks through the egg-shell and sinks 

 to the bottom of the water, where it moves about sluggishly and 

 awaits the arrival of the right host in which to take up its 

 abode. This host is the larva of the Alder-fly, Sialis lutaria Lin. 

 (vide vol. v. p. 444), and into this it bores and comes to rest in 

 the muscles or the fat body. It does not form distinct capsules. 

 It remains in this larva during the following winter, and 

 in the spring passes over into the imago Sicdis. The com- 

 plete insect frequents the small plants growing along the water's 

 edge, and falls an easy prey to the predaceous beetle PL niger. 

 The larva is eaten, and undergoing a change becomes the second 

 larval form mentioned above. It remains in the body of the 

 beetle during the second winter, and finally returns to the water 



1 Zcitschr. wiss. Zool. Bd. vii. 1856, p. 1. 



