142 



KNOWLEDGE 



[August 1, 1891. 



in this earlier stage, the conditions are reversed ; solid food 

 is the order of the day (though plentifully steeped in water, 

 it is true), and no sucking apparatus exists, the mouth 

 being armed with biting jaws instead. 



The change, however, is not suddenly made from the 

 one style to the other. There intervenes a condition in 

 which the insect takes no food at all, either solid or liquid, 

 having no available mouth ; for, when several moultings 

 of the jerky larva have taken place, it makes another 

 change of skin which results in an entire upsetting of aU 

 its arrangements. After this moult it appears as a kind of 



animated " comma," 

 with a big head and 

 a curved tail. The 

 apparent head is really 

 head, thorax, beak, 

 antennse, limbs and 

 wings of the perfect 

 insect, all bound to- 

 gether imder a thin 

 skin, sufficiently trans- 

 parent for the indivi- 

 dual parts to be traced 

 Fig. 2.--Tenimial paddle or leaflet of fl-omwithout,asshown 

 pupa of a species of &nat {Corethra in the photographs 



phimicornis). last month ; the "tail " 



is, of course, the abdomen, and it is terminated by a couple 

 of broad leaf-like paddles (Fig. 2) of exquisite structure, 

 which form a sort of sculUng 

 apparatus. It no longer jerks 

 about head downwards, but, 

 turning a somersault, passes the 

 next stage of its life right way 

 up, notwithstanding its appar- 

 ently top-heavy shape. Con- 

 formably with the altered posi- 

 tion, though whether as cause 

 or consequence it is not easy 

 to say, the opening to the 

 breathing organs is now on the 

 thorax. Two horn-like pro- 

 jections (Fig. 3) are here seen, 

 which are the prolonged lips 

 of the spiracles. Into these is 

 taken, by periodical visits to the 

 surface, whatever air may be 

 necessary for breathing pur- 

 poses ; such visits are, however, 

 by no means frequent, the insect 

 being capable of enduring pro- 

 longed submergence witbout 

 inconvenience. The pupa is as capable of active exertions as 

 was the larva, and in fact is fi-eely locomotive, though it takes 

 no food. This is a most exceptional circimastance amongst 

 insects with a complete metamorphosis. Nothing, moreover, 

 could be in stronger contrast to the style of hfe of the 

 " short-horned " flies than that of this ro\dng Gnat pupa. 

 It will be remembered that the blow-fly, which may be 

 taken as a type of the " short -horns," when about to 

 become a pupa, does not cast its skin, but becomes a 

 barrel-shaped, absolutely motionless body, by the hardening 

 of the last larval coat, whereas the Gnat or Mosquito doe)i 

 cast its skin to become a pupa, and that pupa is a hvely, 

 wriggUng creature, free to wander whither it chooses, though 

 no more capable of feethng itself than the aforesaid barrel. 

 Whep the time for the emergence of the perfect insect 

 arrives, which will be about a month after the hatching of 

 the eggs, the pupa ascends to the surface, and, tipping up 

 its tail, lies in a nearly horizontal position with the back of 



Fio. 3. — Spiraeular horn of 

 same G-nat. — a. Outline of 

 thorax. 



the thorax just above the water. The skin now spUts.and 

 the fly gradually extricates itseH, of com-se in a limp con- 

 dition and incapable of flight till its wings are dried and 

 stiffened. The empty shell of the pupa gives it foothold 

 till it is strong enough to spread its wings and mount into 

 the air for the first time in its life. The occasion of the 

 transformation from pupa to fly is evidently the supreme 

 moment in the Gnat's career, and the risks involved are 

 considerable. Not merely is it still exposed, as it has been 

 hitherto, to the jaws of himgry fish or predaceous water 

 insects, but there are also chances of wind and weather 

 that may prove fatal. However, vast swarms escape these 

 perils and rise into the air, where new dangers await them 

 in the form of cobwebs and insectivorous birds, not to say 

 human beings as well. 



We have now followed our Gnat or Mosquito through a 

 complete cycle of changes, and have thus seen that it is 

 essentially an insect not of the house but of the pond, the 

 marsh, and the swamp, whence it follows that blood- 

 sucking is a practice that can but occasionally be indulged 

 in, and it seems probable that great numbers of Gnats 

 perish without ever tasting such food at all, and that in 

 fact the habit is an acquired one and not really essential 

 to their existence. If this be so, it is all the more remark- 

 able when taken in conjunction with the extraordinary 

 perfection of the blood-sucking apparatus, and the problem 

 of their economy is as difficult to solve as that of the fleas 

 on the sea-shore far from human habitations, to which we 

 referred some time ago. Gnats, however, seem to be quite 

 ready to drink the juices of flowers if they cannot get blood, 

 and several observers have chronicled their fondness for 

 honey. But still this will scarcely explain the presence of 

 needle-like piercers amongst the mouth organs, since such 

 instruments would not be necessary to get at the nectar of 

 flowers. 



In the days when every house had its water-butt, and 

 when stagnant ponds abounded on every side, often in close 

 proximity to human dwellings, the conditions were so much 

 the more favourable for the multiplication of Gnats, and 

 wherever such conditions now obtain, the insects are stiU 

 likely to be both numerous and troublesome. But the 

 extensive abolition of the water-butt, the introduction of 

 closed and indoor cisterns, and the better drainage of the 

 land, have all tended to throw hindrances in the way of the 

 CuUriclic, and have helped to reduce their numbers in our 

 own coxmtry, whatever may be the case elsewhere. There 

 is evidence enough of this in literature. Enormous swarms 

 of Gnats, of one kind or another, seem formerly to have 

 been a not unusual experience, though such a thing now 

 scarcely ever occurs here. The poet Spenser, for example, 

 mentions as a familiar sight " a swarme of Gnats at even- 

 tide " that " out of the fennes of Allan doe arise," 



" Whiles in the air theii- clust'i'ing army flies. 

 That as a cloud doth seem to dim the skies " : 



and that Culices are intended seems certain, since they 

 persecute man and beast 



" Till the fierce northern ivind flith blust'ring blast 

 Doth blow them quite away, and in the ocean east." 



There are several records of swarms that have looked in 

 the distance hke clouds of smoke, and have consequently 

 given rise to an alarm of fire, as was the case at Salisbury 

 Cathedral in 1736. According to Prof. KUey, the northern 

 Mosquitos of America pass the winter in the perfect state, 

 hybernating in a semi-torpid condition, and a writer in hisect 

 Life describes an enormous congregation of them as having 

 been found hybernating in the corner of a cellar. This 

 habit does not appear to hold good in all parts of the world. 

 A very peculiar connection between human beings and 



