294 



♦ KNOWLEDGE ♦ 



[Oct. 10, 1884. 



for the protective case. The head and front segments, 

 however, are hard and horny, as tliey are the only parts 

 exposed when the creature is crawling. The head carries 

 a pair of stout jaws, oftfn notched at the tips. To the 

 three segments immediately succeeding are articulated 

 three stout pairs of legs, which have a wonderfully tenacious 

 grip. To different parts of the body are attached in some 

 species isolated threads, and in others bundles of the same, 

 which are respiratory in function, i.e., they contain branches 

 of the tracheae, and the interchange of gases is effected in 

 the same way as previously described in the case of the 

 dragon-ily larva'. At the end of the abdomen of those that 

 construct movable dwellings, there are two short, recurved 

 hooks, by means of which the case is kept in position and 

 ■dragged along. The larval stage is the great feeding-time ; 

 the insect takes no nourishment during pupahood, and 

 probably very little in its adult stage, its only business 

 then being the reproduction of its kind. But the larvM 

 seem to be able to endure prolonged fasting, and it would 

 appear that they must pass the winter almost without food. 

 After some months sjient in the larval condition, the time 

 for pupation arrives. The two ends of the case must now 

 be closed sufficiently to guard the helpless being within 

 from foes, but, at the same time, not so closely as to prevent 

 the access of water for breathing purposes. Some species 

 construct at each end a sort of grating of silken threads, 

 others fix a quantity of vegetable debris in the same posi- 

 tion. Some take a further precaution still. There is one 

 called Micropterna seqiiax, which inhaljits clear running 

 water. This insect, before pupating, elongates its case by 

 adding stones to one end, and then sinks it vertically 

 in the mud, until it is almost entirely imbedded. To do this 

 the larva turns round in its case, a gymnastic feat no doubt 

 difficult of performance, but still rendered possible by the 

 flexibility of its body and the dimensions of the case — and, 

 thrusting its head and legs out at the wrong end, digs a 

 hole and so lets itself down ; this done, it resumes its ordi- 

 nary position and patiently awaits its coming change. In 

 ■three or four days the pupal stage is entered, and the 

 creature is thereby much altered in appearance. It is no 

 longer a caterpillar-like being ; but all the organs of the 

 adult insect appear — wings, legs, and antennre being neatly 

 folded down by the side of the body, each wrapped in a 

 separate portion of the pellicle which enshrouds the whole 

 creature. The peculiar arrangement of its limbs gives it 

 a most amusingly sanctimonious expression. It is generally 

 free in its case, though its only movements consist of oscil- 

 lations of the body. When the time arrives, some two or 

 three weeks after, for making its final transformation, it 

 ruptures the grating at the larger end of its case, darts out 

 of its prison cell with great speed, swims rapidly through 

 the water by aid of its still encased legs, and on its back, 

 like a water boatman, and thus makes its way to some dry 

 place, where its thin pupal skin splits and allows the soft 

 imago to creep out ; some, however, do not take the 

 trouble to leave the water, but, like gnats, merely float on 

 the surface, and efl'ect their transformation there, using the 

 old pupa case as a raft on which to dry their wings. 



Caddis worms are particularly careful not to expose 

 more than the well-armed part of their body whUe walking, 

 and even then, if an intruder appears, they will instantly 

 and sharply retreat into their cases, when the only part 

 exposed to attack is the hard head, and even this is not to 

 be reached without entering the case. But, notwithstand- 

 ing all their precautions, they do not succeed in escaping 

 the attacks of those inveterate foes of insect life, ichneumon 

 flies, and from one species also has been bred a two-winged 

 fly belonging to a group well known for their habits of 

 parasitism. 



Though caddises are, as a group, aquatic in the larval 

 state, there is one sjjecies the larva of which lives in moss 

 at the roots of trees far removed from water. Nor are the 

 insects absolutely confined to fresh water. One marine 

 species has been reported from North America, and 

 another from New Zealand, the latter of which formed 

 a straight tubular case of fragments of coralline sea- 

 weed. 



We may here notice some allied insects, the 

 larvae of the Ejihemeridce or Mayflies. These crawl- 

 ing things, which may easily be recognised by 

 the three bristle-like appendages at the tail, and a 

 number of leafy projections at the sides, do not make 

 cases, but burrow in the muddy banks of ponds and 

 streams, and constitute what is known to anglers as " bank- 

 bait." The burrows are tubular, and, after running straight 

 for a little distance, bend back upon themselves in the form 

 of a (J, and open into the water at both ends, so that the 

 insect has no need to turn in its burrow, but can enter at 

 one end and make its exit at the other. Small though they 

 are, they are said to live two or three years in the larval 

 condition, a remarkable contrast to the extreme brevity of 

 their adult life, which is measured by hours, or at most by 

 days. The pupa is similar to the larva, except that it ex- 

 hibits traces of wings. When about to change into the 

 winged form, it quits the water and " shuffles off its mortal 

 coil," after the manner of several other insects already 

 referred to ; having so done, it looks like a perfect insect, 

 and might fairly be exjsected to be such, but, marvellous 

 to relate, it has yet another change to undergo — another 

 skin to cast — before it reaches maturity. This is remark- 

 able as being the only instance in the whole class of insects 

 in which a change of skin is effected after the assumption 

 of the winged form. So perfect is this last rejected vest- 

 ment, when left sticking to the stalk or leaf which formed 

 the disrobing place, that, like the cast armature of a cray- 

 fish or lobster, it might easily be mistaken for the complete 

 animal itself. 



(To he continued.) 



THE INTERNATIONAL HEALTH 

 EXHIBITION. 



XIX.— THE SOFTENING OF WATEE— (co«cZ«d«d). 



IT now remains for us, in conclusion of our notes on the 

 softening of water, to give an outline of the process 

 devised by Mr. P. A. Maignen, which has been adopted by 

 the Executive Council, and may be seen in operation at 

 the aquarium tanks of the Exhibition. 



The apparatus employed may be adapted to the wants of 

 the household, or to the treatment of vast quantities. Fig. 



33 is an explanatory drawing of the instrument now in use 

 at the Exhibition. The upper left-hand corner of the 

 engraving shows an automatic reagent precipitator. Fig. 



34 is a diagrammatic drawing to represent the mode of 

 action of this ingenious contrivance. It may there be seen 

 that the water from the main works a paddle-wheel, which, 

 in its turn, causes a revolution of the feed arrangement. 

 Provision is thus secured for a supply of the reagent to 

 the water to be treated, in proportions exactly suited to 

 the requirements of each individual instance. The modified 

 water is somewhat turbid through the precipitation of the 

 lime salts which cause the hardness ; it is, therefore, con- 

 veyed to an asbestos filter shown at the right-hand side of 

 Fig. 33, and this filter is constructed upon the principle of 

 the " Filtre Rapide." Thereafter the water, deprived of 



