422 



♦ KNOWLEDGE . 



[Nov. 21, 1884. 



tlie other segments, but iu the Perlidw (and this is a point 

 which will help to distinguish them from caddis flies) all 

 three segments are very plainly visible, the prothoi ax being 

 quite as large as the other parts ; and, in fact, when com- 

 pared with most other insects, a stone fly seems as though 

 it had three distinct thoraxes. This separateness of the 

 thoracic segments, taken together with the rudimentary 

 condition of the mouth organs, and the fact that no 

 quiescent pupa stage intervenes between the larva and 

 perfect insect, indicates lowness of type. 



The fore-wings are long and narrow, but the hind pair 

 broader and more nearly triangular. In most insects the 

 points of attachment of both pairs -of wings to the thorax 

 are, iu consequence of the fusion of the thoracic segments, 

 closely approximated ; but in the Ptrlid(t this is naturally 

 not the case, and the wings at their basfs not only do not 

 overlap, but are separated by a considerable interval, an 

 arrangement that suggests the idea of parts of two different 

 insects having been joined to make the complete creature. 



In their larval form, most of the PerlidiK prefer swiftly- 

 running streams, at the bottom of which they conceal 

 themselves under stones, likc. Like the caddis flies, the 

 female carries her eggs for a time at the end of the 

 abdomen, as a little black globular bundle. The larvse 

 are very similar in shape to the adult insects, minus 

 wiugs, and, unlike the caddis worms, do not construct 

 cases for defence. They are carnivorous, preying upon 

 the larvaj of may-flies and other aquatic insects, but, not- 

 withstanding this, they are neither particularly active, nor 

 endowed with remarkably good ofiensive weapons. They 

 therefore secure their prey by the expedient of ambus- 

 cade, lurking behind stones or pond-weeds in order to 

 surprise their victims, and even sometime.s throwing a 

 little mud over themselves, in order to render their con- 

 cealment the more efl'ectual. This latter habit is practised 

 principally by those whose bright colour would otherwise 

 render them too conspicuous on the dingy muddy bottom 

 which forms their hunting-ground. The pupa resembles 

 the larva, except that it shows traces of wings When the 

 time comes for its final change, it leaves the water and 

 wanders about in the neighbourhood till it finds a place 

 suitable for its disrobement ; such a spot will be the sur- 

 face of a rock, or any other material that, by its roughness, 

 aflbrds a good foothold, without which it would be unable 

 to extract its limbs from their encasing skin. Having 

 chosen the site, it takes a good grip of the irregularities of 

 the surface, by means of its tiny claws, and then, as with 

 so many other insects, the skin splits along the back of 

 the neck, and the fully-formed being, which has already 

 become loosened from its skin by the passage of air under 

 the latter, gradually extricates itself from its investing 

 pellicle. 



The flight of a stone-fly is heavy and direct, the weak- 

 ness of its wings preventing it from being able to " cut 



Nemoura variesata. 



capers " in the air. The slightest; touch is sufficient to 

 arrest its flight and precipitate it to the ground. But 

 this is no great disadvantage, for its winged life is of short 

 duration, lasting only a few days, the rudimentary character 



of the mouth organs rendering the taking of nourishment 

 impossible. Fig. 1 represents one of the commonest of 

 our British stone-flies, and one, too, which inhabits stagnant 

 water as well as running streams. It is sometimes called 

 the willow-fly. 



The Alder-fly or Orl-fly is called Slalis lutaria. It 

 belongs to quite a difltrent group from the preceding 

 insect, viz., the Neuroptera Planipennia, a secti<.n con- 

 taining about fifty British species. It Ls a lazy, blackish 

 creature, with a large number of extremely thick and dark 

 nervnres on its smoky wings, whereby it may at once be 

 recognised. There are plenty of insects with as many and 

 even more nervures, but none with them so thick and 

 conspicuous. In May the flies are often to be seen in pro- 

 digious numbers, sitting about on paling.s, stones, <tc., near 

 a pond. The female deposits an enormous number of eggs, 

 which she is said to lay one by one on rushes or other 

 aquatic plants ; they are attached side by side with great 

 regularity, and may be recognised by being terminated at 

 the top by a little pointed projection. They are generally 

 deposited in the immediate vicinity of water, but occa- 

 sionally at a distance therefrom that must necessitate a 

 considerable terrestrial journey on the part of the newly- 

 hatched larva before it can become properly domiciled^ 

 The larva has its respiratory appendages reduced to slender 

 filaments ; seven or eight pairs are attached to the sides 

 of the abdomen, and by their rowing motion the creature 

 propels itself. At the close of its preliminary stages, it 

 quits the water, and, unlike the stone-flies, burrows in the 

 neighbouring bank, and forms a cell in which to pupate. 

 The pupa, though incapable of transporting itself from 

 place to place, yet has considerable powers of movement^ 

 and vigorously twists its tail about when disturbed. 



A B 



Fig. 2. — Fore-wiDg of Sialis Intaria (A) and S. foliginosa (B)". 



There are but two British species in the genus Sialis, 

 They are very much alike, and would be undistinguishable 

 to any but the expert, who knows just what differences to 

 look for. Mr. McLachlan has pointed out that there is 

 one little nervure in the fore-wings (Fig. 2 a), the position 

 of which, relatively to the surrounding ones, varies in the 

 two species ; but is so constant in each as to atTord the 

 easiest way of distinguishing them. Of course, this is not 

 the only difiTerence, or it could scarcely justify the separation. 

 of the species, but the others are less easy of recognition. 

 (To fee coniinved.) 



AUTOMATIC VENTILATION, SO- 

 CALLED. 



Now that the question of the public health, and all 

 matters appertaining thereto, are being brought into 

 such great prominence, it may, perhaps, not be uninteresting 

 to our readers if we attempt to dispel the " magic halo 

 with which would-be sanitary experts endeavour to obscure 

 the actions of some of the chief agents concerned in auto- 

 matic ventilation. In so doing, and even if we have occa- 

 sion to allude to the various physical laws which regulate 

 the results obtained from systems of automatic ventilation, 

 we shall endeavour to keep always before us our -watch- 

 word, " Science plainly worded, exactly described." 



