404 



ANIMAL RESPIRATION 



during the early part of their existence, and, although land-animals 

 when adult, are always found in the neighbourhood of water. 

 There is no quiescent or pupa stage in the life-history, but the 

 young Stone- Fly, when hatched from the egg, closely resembles 

 the adult, except that wings are absent. It is usual to apply the 

 term nymph to a stage of this sort, reserving the word larva for 

 caterpillars, grubs, and the like, that are very unlike the perfect 

 insects which they become after passing through a more or less 

 quiescent pupal condition. 



The nymphs of Stone- Flies possess numerous air-tubes, but 

 these do not open to the exterior by means of spiracles. Breath- 

 ing is either effected through the skin, 

 which is very thin at certain spots, or by 

 means of tracheal gills, which differ in 

 shape and position. These gills persist 

 in the adult (as, e.g. in Pteronarcys\ 

 though they often become reduced in 

 size, and it is not known how far they 

 are of any use (fig. 568). That they 

 should persist at all is very interesting in 

 view of the fact that in certain extinct 

 forms (see p. 463) they were possessed 

 by the perfect insect. We know that 

 the coal-plants, among which these an- 

 cient insects lived, grew in jungles and 

 swamps where the air was very damp, 

 and this probably favoured the retention 

 of tracheal gills throughout life. 

 Dragon- Flies (Odonatd). A female dragon-fly lays her eggs 

 either in the water or upon the stem of a water-plant, and they 

 hatch out into flattened wingless nymphs, which possess an 

 elaborate system of air-tubes that probably do not communicate 

 with the exterior. The way in which such nymphs breathe is 

 not always the same, but the commonest, and at the same time 

 the most interesting, is by means of rectal gills, so called because 

 they are folds in the lining of the last part of the intestine 

 (rectum]. These folds are either plate-like or in the form of 

 small finger-shaped projections, but in either case they are richly 

 provided with air -tubes, and collectively possess a very large 

 breathing surface (fig. 569). The rectum alternately contracts 



Fig. 568. A Stone-Fly (Pteronarcys] 

 A, Under side of adult, with limbs cut 

 short ; gggg, reduced tracheal gill-tufts 

 (enlarged); B, a gill -tuft (more highly 

 magnified). 



