252 AETHROPODA [CH. 



This order includes the Insects familiarly known as Dragon-flies. 

 Two of the best known genera are Libellula and Aeschna (Fig. 74). 



A number of other small orders of Insects agree with the Para- 

 neuroptera in possessing two pairs of membranous wings with 

 numerous nervures. These were formerly grouped with the Para- 

 neuroptera in one large order termed the Neuroptera and this 

 course was adopted in former editions of this book. Continued 

 research has shown that amongst these "nerve-winged" insects 

 profound differences occur both as to the character of the mouth- 

 parts and the nature of the metamorphosis ; and in consequence 

 the old group of the Neuroptera has been divided up, and its 

 divisions given the rank of orders. A few only of these orders 

 will be mentioned here, viz.: 



Order IV. Ephemeroptera or Day-flies. 



In these Insects the nymph stage is passed in water and lasts a 

 long time, in some cases as much as two years. Breathing is 

 effected by the so-called tracheal gills wing-like organs projecting 

 from the sides of the abdomen in which tracheae ramify. These have 

 been recently shown to be modified abdominal appendages. Oxygen 

 diffuses from the water into the air in these closed tracheae and 

 thence into the blood, instead of as in the gills of animals of true 

 aquatic ancestry, directly into the blood. From this circumstance 

 it is clear that the nymph represents a terrestrial not an aquatic 

 ancestor. The imago takes no food, it emerges from the cuticle of 

 the nymph in the evening and hovers over the water for a few hours 

 during which time mating and laying of eggs is accomplished. The 

 gnathites though vestigial are of the biting type. Ephemera, the 

 May-fly, is the commonest British form. 



Order V. Isoptera, Termites or White Ants. 



In these Insects the gnathites are like those of the Cockroach, of 

 a strongly marked biting type. They live in large colonies which 

 include one perfect sexually developed female (the queen) and one 

 ripe male at least, and a very large number of imperfectly developed 

 wingless females which are divided into two varieties, termed 

 respectively the workers with small heads and the soldiers with 

 enormous heads and powerful jaws. They live on decaying wood 

 and burrow in it, never exposing themselves to the light if it can be 



