424 NEUROPTERA CHAP. 



functionless ; he appears to be of opinion that when the first 

 thoracic stigma is closed this is the result of the abutting against 

 it of a closed trachea. Dewitz found l that in the adult nymph 

 of Aesclina the thoracic stigma is well developed, while the other 

 stigmata to what number and in what position is not stated 

 are very small. In a half-grown Aeschnid nymph he found the 

 thoracic stigma to be present in an undeveloped form. On placing 

 a full-grown nymph in alcohol, gas escaped from the stigma in 

 question, but in immature nymphs no escape of gas occurred 

 although they were subjected to a severe test. A specimen that, 

 when submitted to the above-mentioned immersion, emitted gas, 

 subsequently moulted, and thereafter air escaped from the 

 spiracle previously impervious. The observations of Hagen and 

 Dewitz are perhaps not so adverse to the views of Palmen as has 

 been supposed, so that it would not be a matter for surprise if 

 Palmen's views on this point should be shown to be quite correct. 



The number of species of Odonata or Libellulidae that have 

 been described is somewhat less than two thousand, but constant 

 additions are made to the number, and when the smaller and more 

 fragile forms from the tropics are collected and worked out it will 

 probably be found that the number of existing species is somewhere 

 between five and ten thousand. They are distributed all over 

 the world, but are most numerous in species in the warmer regions, 

 and their predominance in any one locality is very much regulated 

 by the existence of waters suitable for the early stages of their 

 lives. 



A good work on the British Odonata is still a desideratum. 2 

 In Britain about forty-six species are believed to be native. They 

 are said to be of late years less numerous than they used to be. 

 Notwithstanding their great powers of flight, dragon-flies are 

 destroyed by birds of various kinds ; several hawks are said to be 

 very fond of them, and Merops persicus to line its nest with their 

 wings. The number of Insects killed by dragon-flies in places 

 where they are abundant must be enormous ; the nymphs, too, 



1 Zool. Anz. xiii. 1890, p. 500. 



2 The following works convey the best information : Evans's British Libellulinae or 

 Dragon-flies, illustrated in a series of lithographic drawings, 1845. Hagen, "A 

 Synopsis of the British Dragon-flies," in Entomologists' Annual, 1857. M'Lachlan, 

 Catalogue of the British Neuroptera, published by the Entomological Society of London 

 in 1870; and "The British Dragon-flies annotated," Entom. Month. Mag. xx. 1884, 

 pp. 251-256. 



