THE OPEN TYPE OF WING-GROWTH 77 



compound eyes, the flattened abdomen with elongate tail- 

 appendages that characterize the adult ; by means of its 

 strong mandibles it catches and bites up small insects which 

 serve it as prey. Living under water it needs some special 

 adaptation for breathing, and this is found in six pairs of tufted 

 tubular gills, situated on the sides of the thorax close to the 

 bases of the legs and at the junction of adjacent segments. 

 These gills are connected with the trunks of the air-tube system. 

 Thus through a series of moults the young grows on towards 

 its goal, showing outward wing-rudiments at an early stage, 

 and finally crawling out of the water so that it may cling to 

 a stone and allow the fly to emerge into the air where it can 

 expand and dry its wings. With the development of these 

 there is as among the winged aphids an elaboration of the 

 second and third thoracic segments. The fly's jaws are weak 

 compared with those of its aquatic young, as it lives but a 

 short time and takes but little food. An interesting survival 

 from the early life under water is seen in the presence on some 

 stone-flies of the tufted tracheal gills in a withered and reduced 

 state. 



Some of the stone-flies also resemble aphids as well as cock- 

 roaches in a curious and interesting diversity in the size of the 

 wings. In several species it is quite usual to find individuals 

 with wings less than half the normal size, and this tendency 

 to shortening is carried farther among males than among 

 females a sexual modification uncommon among insects, and 

 opposite to that shown by the aphids and cockroaches in which 

 abbreviation or disappearance of the wings is more often a 

 female than a male character. 



The great order of sucking-insects to which aphids belong 

 comprises other families in which the young throughout their 

 period of growth, display a striking likeness to their parents. 

 We may take for example, a family of plant-bugs (Capsidac) 

 which, like the aphids, feed by sucking sap from plant-tissues, 

 some of them causing at times serious damage to foliage and 

 young fruits in garden and orchard. An adult capsid plant-bug 

 (Fig. 42 g) has a broad, bluntly triangular head with laterally- 

 situated compound eyes and feelers which, although elongate, 

 consist of only four segments, the basal one being stout and 

 the other three long and slender Below the head is the 



