AMPHIBIOUS INVERTEBRATES 467 



hatch out from them may be called larvae (not nymphs), because 

 they differ considerably from the adult, into which they do not 

 pass directly, there being an intermediate quiescent or pupa stage. 

 Each of the first seven rings of the abdomen bears a pair of 

 tracheal gills in the form of jointed threads, and an unjointed rod 

 projects backwards from the tip of the tail. The gills are kept in 

 constant motion, so that the water in their vicinity is continually 

 renewed. 



Caddis- Flies {Phryganeidce). — An adult Caddis- Fly has four 

 somewhat hairy wings, giving it some resemblance to a moth; 

 indeed, it is sometimes called a Caddis-" Moth ". The eggs are 

 laid in water, and from them elongated larvse hatch out, which 

 are familiarly known as " caddis-worms ", and construct for them- 

 selves protective cases of such materials as may be at hand. 

 The cases of the commonest British species are straight or 

 slightly curved tubes, with a large opening at the head-end and 

 a smaller one at the tip. The breathing organs of the larva 

 are thread-like tracheal gills borne by the abdomen, upon the 

 upper side of which are also a number of thin places which 

 possibly assist in respiration by permitting diffusion of gases. 

 The gills are kept moving, and water continually streams through 

 the case, entering at its hinder end. 



AMPHIBIOUS TWO-WINGED INSECTS (Diptera) 



In two families of this order the early stages of existence 

 are fully adapted to life in water, while the adults are aerial and 

 breathe ordinary air by means of a complex set of air-tubes. 

 These are Midges {ChironomidcB) and Sand-Midges iSimuliadcs). 



Midges (jOhironomidcs). — A very large number of Midges are 

 included in the genus {Chironomits^ from which this family takes its 

 name. The eggs are laid in water, and hatch out into active wrig- 

 gling larvae, which are commonly of a red colour, and are then 

 known as " blood-worms ". This hue is caused by the presence of 

 a pigment {haemoglobin) identical with that contained in blood, and 

 which is of great importance in breathing, since it readily takes up 

 oxygen from the surrounding medium, easily parting with it again 

 to the body. Some of these larvae live at considerable depths 

 in lakes (1000 feet in Lake Superior) or even in the sea, and 

 the amount of red colouring-matter they possess is in direct pro 



