178 THE ENTOMOLOGIST. 



Dytici cannot come there to seek a fresh provision of atmospheric 

 air ? Must we admit that they have the faculty of living and of 

 moving during very long periods, while consuming only the pro- 

 vision of air that they have stored up ? A number of terrestrial 

 insects, it is true, resist asphyxia for some time when they are 

 kept submerged under water. They consume the air stored up 

 in their tracheal, and, as they are then more or less torpid, their 

 respiration is less active, and, correspondingly, their provision of 

 air is exhausted more slowly. But, to be exact, this resistance 

 lasts for a few days at the most, and there are many ponds and 

 ditches containing Dytici where the surface remains entirely 

 frozen over for weeks. I was anxious to experiment upon the 

 resistance of D. marginalia against asphyxia. I therefore dis- 

 posed, at different times, some male and female Dytici in such a 

 manner that they could swim freely, but, by means of a wire- 

 grating, were always submerged. My experiments show that in 

 general — there are, of course, very numerous, but more or less 

 insignificant individual variations — the Dytici which cannot 

 come to the surface to renew their store of air are, at tie end of 

 one to three days, more or less torpid, move slowly, sometimes 

 even appear dead, and, after a duration of two to five days 

 (according to the endurance of individuals), they have very 

 generally ceased to live. Eight days are, I believe, the maximum 

 of the duration of resistance of D. marginalis against asphyxia. 



I observed that if the elytra and wings of Dyticus are removed, 

 or if only the posterior half of the elytra and wings are cut away, 

 the insect ascending freely to the surface of the water, it usually 

 dies at the end of one to three days, for in this condition the air 

 penetrates only very insufficiently, or even not at all, into the 

 tracheae. On the other hand, the duration of resistance against 

 the want of oxygen is prolonged when one removes from each 

 individual the elytron and wing on one side only. In fact, the 

 insect, on returning to the surface, stores up a provision of air 

 under the remaining wing and elytron ; but this is done only 

 with difficulty, and the respiration operates more or less badly, 

 and always so inefficiently that death ensues at the end of a 

 few days. 



One can, I believe, deduce from these experiments that in the 

 ponds and ditches in which the surface is completely frozen the 

 Dytici habitually preserve their normal activity, and that they 

 find the atmospheric air indispensable to their respiration — 

 under the mantle of ice, under the leaves, amid the branches of 

 plants, under stones at the margins of streams, &c. One may 

 suppose also that many individuals lessen their activity in con- 

 sequence of an insufficient quantity of oxygen for their respira- 

 tion ; but I am convinced that if they were completely deprived 

 of atmospheric air, all the Dytici in a pond or ditch would, at the 

 end of eight days, entirely cease to live. 



