XVIII DRAGON-FLIES 425 



are very destructive in the waters they inhabit, so that dragon - 

 flies have no doubt been no mean factor in maintaining that 

 important and delicate balance of life which it is so difficult 

 for us to appreciate. The nymphs are no doubt cannibals, 

 and this may perhaps be an advantage to the species, as the eggs 

 are sometimes deposited in large numbers in a limited body of 

 water, where all must perish if the nymphs did not, after exhaust- 

 ing other food, attack one another. Martin, speaking of the 

 Odonata of the Departement de I'lndre in France, says : ^ " The 

 eggs, larvae, and nymphs are the prey of several fishes, snakes, 

 newts, Coleoptera, aquatic Hemiptera, and of some diving birds. 

 Sometimes the destruction is on a considerable scale, and one 

 may notice the dragon-flies of some piece of water to diminish 

 gradually in numbers, while the animals that prey on them 

 increase, so that a species may for a time entirely disappear in 

 a particular spot, owing to the attacks of some enemy that has 

 been specially prosperous, and also eager in their pursuit. De 

 Selys found that from a pond filled with carp, roach, perch, 

 and eels, several of the dragon-fly denizens disappeared directly the 

 bream was introduced." On the other hand, there can be little 

 doubt that the nymphs are sometimes injurious to fish ; it has 

 been recorded that in a piscicultural establishment in Hungary 

 50,000 young fishes were put into a pond in spring; in the 

 following autumn only fifty-four fish could be found, but there 

 were present an enormous quantity of dragon-fly nymphs. 



Odonata are among the few kinds of Insects that are known 

 to form swarms and migrate. Swarms of this kind have been 

 frequently observed in Europe and. in North America ; they 

 usually consist of species of the genus Lihellula, but species of 

 various other genera also swarm, and sometimes a swarm may 

 consist of more than one species. L. quadrimaculata is the species 

 that perhaps most frequently forms these swarms in Europe ; 

 a large migration of this species is said to occur every year in 

 the Charente inferieure from north to south.^ It" is needless to 

 say that the instincts and stimuli connected with these migrations 

 are not understood. 



The nymphs are capable, under certain circumstances, of 

 accommodating themselves to very peculiar conditions of life. 

 The Sandwich Islands are extremely poor in stagnant waters, and 



1 Rev. cVEntomol. v. 1886, p. 232. - Riveau, Fcuille Nat. xii. 1882, p. 123. 



