THE INSECTS. 325 



others are far longer lived, though the extreme limit is 

 probably rarely more than a week. These insects are all 

 aquatic in their preparatory state. The eggs are dropped 

 into the water by the female in large masses, and it is 

 probably several months before the larvae are excluded. 

 The sub-aquatic condition lasts a long time in the genus 

 Ctoeon, a small and delicate species, Sir John Lubbock 

 having proved it to extend over six months ; but in the 

 larger and more robust genera (e.g., Planigenia) there 

 appears reason to believe that the greater part of three 

 years is occupied in the preparatory conditions. 



" The larva is elongate, the head rather large, furnished 

 at first with fine, simple eyes of nearly equal size ; but 

 as it increases in size the homologues of the faceted 

 eyes of the imago become larger, whereas those equivalent 

 to the ocelli remain small. The antennae are long and 

 thread-like, composed at first of a few joints, but the 

 number of these latter apparently increase at each moult ; 

 mouth-parts well developed ; powerful mandibles ; three 

 distinct and large thoracic segments ; the proto-thorax 

 narrower than the others ; legs much shorter and stouter 

 than in the winged insects ; abdomen, ten segments, the 

 tenth furnished with long and slender multi-articulate tails 

 (setae), which appear to be only two in number at first, 

 but an intermediate one gradually develops itself (though 

 the latter is often lost in the winged insect). Respiration 

 by external gills, placed along both sides of the dorsum 

 of the abdomen and hinder segments of the thorax. 

 According to Lubbock and Joly, the very young larvae have 

 no breathing organs, respiration being effected through the 

 skin. Lubbock traced at least twenty moults in Cloeon. 

 At about the tenth, rudiments of the wing-cases began to 

 appear. These gradually become larger, and when so the 

 creature may be said to have entered its nymph stage ; 

 but there is no condition analogous to the pupa state of 

 insects with complete metamorphoses. There appear to be 

 three or four different modes of life in these larvae some 

 are fossorial, and form tubes in the mud or clay in which 

 they live ; others are found on or beneath stones ; while 



