MAY-FLIES 169 



half of the winter they are popular microscopic objects ; after 

 Christmas they become too large and opaque to be altogether 

 convenient. The up-and-down movements of the gills are the 

 chief point of interest. There are seven pairs of them, borne 

 on as many abdominal segments. Each gill is forked, and 

 most of them are fringed with long filaments. The first has 

 no fringe, while both the first and the last are smaller than the 

 rest, and fixed, performing no respiratory movements. These 

 gills are traversed by trachea, or air-tubes, and are hence called 

 tracheal gills. Ordinary gills, such as those of a fish, contain 

 no gaseous air; it is the blood which becomes aerated, and 

 oxygen circulates in the body only when dissolved. But in 

 tracheal gills gaseous oxygen, extracted from the freshly aerated 

 blood, is stored in the air-tubes. When full-fed, the larva is 

 not much less than an inch long (much more, if its antennae 

 and tail - filaments are included). It has three pairs of long 

 and many-jointed legs, suitable to an insect which has often 

 to creep over the rough floor of a stream or among the leaves 

 and twigs of a ditch ; each foot ends in a single pointed claw. 

 On the back of the thorax is a dark, lozenge -shaped mark, 

 distinctive of this particular larva ;. most of the abdominal 

 segments are more or less concealed by the gills, but the last 

 two or three are exposed. From the extremity of the abdomen 

 stand out three tail-filaments, which are jointed and fringed ; 

 these seem to be used as accessory breathing organs, for they 

 are freely supplied with blood, pumped into them by a special 

 chamber of the heart. In certain Ephemerid larvae they are the 

 chief or only swimming organs. On the head we see a pair of 

 rather long feelers, three simple eyes, and apair of compound eyes. 

 The mouth-parts are adapted to biting, and generally resemble 

 those of Orthopterous insects. The mandibles are extremely 

 long and pointed ; when at rest, they are crossed, and extend 

 beyond the head. Such mandibles as these are usual in pre- 

 datory insects, and the common May-fly feeds largely, though 

 not exclusively, upon other aquatic animals. Near the base 

 of the mandible are stout hooks and a ridged grinding surface. 

 The larva creeps, swims, or burrows, as occasion requires. It 

 is much preyed upon by other insects, and is, like Ephemerids 

 of all kinds, a favourite food of fishes. When it begins to 

 approach its full size, the peculiar organs of the fly, such as 

 the wings and the claspers of the male, protrude from the 



