334 BIONOMICS, ETC. [CH. 



Drought. 



It is a widespread belief that the appearance of a swarm of 

 Dragonflies heralds a drought. There is some truth in this. 

 Observations which I have made on the drying-up of water-holes 

 in the Sydney district lead me to conclude that, of all the larger 

 aquatic life in the pond, the Dragonfly larvae will last out longest. 

 As the volume of water decreases, the amount of food to be 

 obtained with ease increases, so that the larvae take toll of every- 

 thing that conies to hand. Thus, if they mature before the final 

 drying-up of the pond, a huge swarm of Dragonflies will emerge, 

 and precede the worst part of the drought. The other side to the 

 picture is not so likely to .be noticed, but it is this : In the spring 

 of 1908, the pond I was observing dried up early in September, 

 whereas it usually lasted until December or later. On the hot 

 caking mud of the pond, I counted over three hundred nearly 

 full-fed larvae of Anax papuensis. Every one of these perished. 

 In a week or less, had the water held out, they would have appeared 

 as a huge sw r arm of Dragonflies, "heralding" a drought that lasted 

 for several months ! 



The larvae of certain Dragonflies can resist prolonged drought. 

 In the case of Synthemis [170] I have kept the larva in dry sand for 

 three months. At the end of this period they were apparently 

 dried up, and so light that, when placed in water, they floated ! 

 But they soon revived, and, after some struggling, forced 

 their way to the bottom, where they at once burrowed in the 

 usual way. Several of these were bred out later on. The larvae 

 of Austrocordulia can also resist drought, as probably also can 

 those of many of the Sympetrini. But lack of water is fatal to most 

 Zygoptera and Aeschninae. The peculiar larva of Telephlebia has 

 a slimy integument, and prefers to live out of the water rather 

 than in it [178]. 



Economic Value of Dragonflies. 



The Odonata appear to touch the life and activities of man 

 only very lightly. Were they to disappear from the earth, most of 

 us would probably not feel their loss in the slightest. On the other 

 hand, there are so many injurious and obnoxious insects about 

 us at every turn, that we may take a pleasure in contemplating 

 the fact that the Dragonflies are a beneficent and not a maleficent 



