46 British Dj'cigonflies. 



— the Anisoptcridcs — they are contiguous (not, however 

 in the GonipJiina;) to a greater or less extent along the 

 middle line of the head, but in the other division — the 

 Zygoptcridcs — in which the head is transversely broad, or 

 hammer-shaped, they are separated to a considerable 

 degree. The number of hexagonal facets is very large, 

 and in many cases the upper facets are of a greater size 

 than the lower ones, this difference in size being prob- 

 ably co-ordinate with a difference in function. It has 

 been suggested that the lower facets are suited for 

 seeing objects near at hand, while the upper ones are 

 used when the insect is on the look out for foes or 

 food. Dragonflies possess also three ocelli, or stcmniata 

 {i.e., rounded simple eyes). In the Zygopterides they are 

 placed in a triangle on the top of the head between the 

 compound eyes ; but in those species in which the latter 

 occupy too much space there is a prominence in front 

 of the line of contact, and two of the ocelli are placed 

 on this, one on each side, while the third is in front 

 and sometimes much hidden by the prominence. 



The larger, swiftly-flying Dragonflies are able to see 

 to a considerable distance, as witness the ease with 

 which they sight one of their fellows, or some other 

 insect, some dozen yards away, that appears fit to eat. 

 Their sight, however, does not seem to be very perfect, 

 for they often approach very close to another Dragon- 

 fly before discovering that it is of a different species, 

 and not seldom turn away from an insect when quite 

 close upon it, having discovered probably that it is 

 not suitable for food or too dangerous to attack. They 

 are readily startled by a shadow falling upon them, and 

 by one's rapid approach, but thc\' do not seem to notice 



