INSECTA. 601 



future limbs begin to show themselves under the form of 

 transparent vesicles, which, as they enlarge, crowd the 

 thoracic region of the body. The change from the larva 

 to the pupa condition involves phenomena of the most 

 startling character. The air- sacs, situated both in the 

 thoracic region and in the hinder portion, burst and 

 unfold themselves into an elaborate tracheal system, and 

 a pair of ear-shaped tubes, of which not the slightest trace 

 could hitherto be discerned, make their appearance upon 

 the dorsal aspect of the thorax ; two long tracheae seem to 

 be thus simultaneously produced, occupying the two sides 

 of the body, and constituting the main trunks, from which 

 large branches are given off to supply, in front, the head, 

 the eyes, and the nascent limbs ; while posteriorly they 

 spread over the now conspicuous ovaries, and terminate by 

 ramifying largely through the thin lamellae that constitute 

 the caudal appendages. In individuals subjected to micro- 

 scopic examination within a very brief period after their 

 assumption of the pupa state, the places originally filled 

 by the air-sacs of the larva are found to be occupied by 

 the lateral remnants of their external coats, clearly indi- 

 cated by ragged membranes, covered with patches of black 

 pigment, in the immediate vicinity of which numerous 

 air-bubbles are met with, extravasating, as it were, into 

 the cellular tissue. 



The family Phryganeidce, the larvae of which are aquatic, ' 

 present almost as little resemblance to the imago as those 

 of some metabolous insects. They are long, softish grubs, 

 furnished with six feet, and with a horny head armed with 

 jaws, generally fitted for biting vegetable matters, although 

 some appear to be carnivorous. To protect their soft 

 bodies, which constitute a very favourite food with fishes, 

 the larvae are always inclosed in cases formed of bits of 

 straw and sticks, pebbles, and even small shells. The 

 materials of these curious cases are united by means of 

 fine silken threads, spun like those of the caterpillars of 

 the Lepidoptera, from a spinnaret situated on the labium. 

 In increasing the size of its case to suit its growth, the 

 larva is said to add to the anterior part only, cutting off a 

 portion of the opposite extremity. When in motion, the 

 larva pushes its head and the three thoracic segments, 



