478 NEUROPTERA 



At the end of the larval period of existence the creature 

 closes its case by a light web spun at each end, taking care not 

 to prevent the ingress and egress of the water ; it sometimes adds 

 a stone or piece of stick, and having thus protected itself, changes 

 to a nymph. During the first part of this metamorphosis the 

 creature is completely helpless, for there is so great a difference 

 between the external structures of the larva and nymph as to 

 make the latter a new being, so far as these organs are concerned. 

 The changes take place in the interior of the larval skin, and as 

 they are completed this latter is shed piecemeal. The resulting 

 pupa or nymph greatly resembles the perfect Insect, differing 

 consequently very much from the larva. Pictet, who paid 

 special attention to the nymph condition of these Insects, con- 

 cludes, however, that many of the organs of the nymph are 

 actually formed within the corresponding parts of the larva, and 

 has given a figure that, if trustworthy, shows that the legs of 

 the nymph, notwithstanding the great difference between them as 

 they exist in the larva and in the perfect Insect, are actually 

 formed within the legs of the larva ; each nymphal leg being 

 rolled up in the skin of the corresponding larval leg, in a 

 spiral, compressed manner, and the only articulations that can be 

 detected in the leg being those of the tarsus. The head of the 

 nymph is armed in front with two curious projections that are, 

 in fact, enormously developed mandibles (Fig. 323,B); they serve as 

 cutting implements to enable the nymph to effect its escape from 

 its prison ; they are cast off with the nymph-skin, the perfect 

 Insect being thus destitute of these organs. The abdomen of the 

 nymph differs from that of the perfect Insect in possessing 

 external respiratory filaments ; the nymphs of some species have 

 also the middle legs provided with swimming-hairs, that do not 

 exist in the imago. 



As a rule the larvae bring the respiratory filaments into con- 

 tact with the water by moving the abdomen, but Fritz Miiller 

 found l that those of a Macronema move the gills themselves after 

 the manner of Ephemeridae with much rapidity. Many kinds 

 of larvae of Phryganeids possess at the posterior extremity of the 

 body exsertile pouches in the form of finger-like, or even branched, 

 processes into which tracheae do not enter. Miiller observed that 

 in the Macronema alluded to these pouches were generally not 



1 Ent. Nachr. xiv. 1888, p. 274. 



