76 INSECT TRANSFORMATION 



appear on the second and third thoracic segments in those 

 aphids that are destined to grow into winged insects, and after 

 the next moult, when the fourth stage is reached, these are 

 prominent, reaching backwards alongside the body as far as 

 the third or fourth abdominal segment. After the fourth moult 

 the adult condition is attained, and the winged aphids then 

 display the hardened thoracic exoskeleton adapted for the 

 attachment of the organs of flight. In the development of 

 the wingless generations there is the same number of stages, 

 but no trace of wing-rudiments, and the young naturally 

 resembles the wingless more closely than the winged adult. 

 The intermediate aphids, with reduced wings, already men- 

 tioned, are indistinguishable from their fully-winged sisters 

 during the third and fourth stages, but, when adult, they 

 resemble these less closely than they do the wingless forms, 

 as the thoracic cuticle remains thin and the exoskeletal struc- 

 tures are unspecialized. The occasional development of these 

 intermediate aphids may give a hint of the process whereby 

 the wingless have become modified out of the winged forms 

 among these insects. In the one case the wings attain but a 

 feeble growth ; in the other they entirely fail to appear. 



The young aphids not only resemble their parents closely 

 in form, but live in similar surroundings and conditions. We 

 may turn next to a group of insects in which while the young 

 is generally like its parent, it lives in very different surroundings 

 in water instead of in the atmosphere. These insects are 

 the Stone-flies, a small order (Plecoptera) allied to that of the 

 cockroaches and grasshoppers (Orthoptera), for one of our 

 larger species of Per la has biting jaws, often much reduced, 

 wings of which the front pair are elongate and narrow and 

 the hinder pair broad with an extensive folding anal area, 

 though the forewing is (unlike that of a grasshopper or cock- 

 roach) of the same texture as the hindwing, and long paired 

 cerci at the tail-end of the body. The head is relatively broad 

 with a pair of long, many-jointed feelers, and the body generally 

 is somewhat flattened from above downwards. These insects 

 are found on the banks of streams or on half-submerged stones, 

 sometimes flying rather heavily for short distances. The 

 female drops her eggs into the water in which the young live. 

 The young Perla has the broad head with long feelers and 



