60 



KNOWLEDGE. 



[Makcii, 1902. 



a stifcossful liuiitcr, rajiiilly to iiltci' its coursu and adapt 

 itscdf to the sudden moveiiiciits of its rjuarry. Tlu-.so 

 iusec'ts are, in fact, adepts at " dodfjin;,'." and liowover fast 

 they may be ilyinj,', can instantly wheel at a sluiqi ani^le, 

 oi' even make a comi)lele voHv face. It can hardly lie 

 doubted tliat the independence of the wings is an important 

 fa<:tor in these movements, so that, while a certain amount 

 of power is lost to the stroke by the fact that the wiut,'s 

 are not linked together, yet this is eom|)ensate(l for by tlie 

 ability to wheel suddenly, which is brought about by the 

 absence of such union. 



In this country we have only a few other insects with 

 wings constructed like those of dragon-flies ; the chief of 

 these are the lovely lace-wing flies, which are amongst the 

 most cTiiiicate and fairy-like creatures we possess. But as 

 they tave ouly a very feeble power of flight, they need not 

 concern us further on the present occasion. 



We pass now to those insects that possess wing-links of 

 some kind or other. Amongst these the Hymeuoptera 

 take tlie first place. The order contains bees, wasps, ants, 

 sawflies, and that host of parasitic insects known as 

 i(.-hueumou flies. These creatures have two pairs of mem- 

 branous wings, the anterior of which are much larger than 

 the posterior. Each wing, whether primary or secondary, 

 is by itself long and narrow, and in proportion to the size 

 of the body, offers but a small surface to the action of the 

 air, and hence would not have much sustaining power. 

 But the anterior edge of the hind-wing has neai'ly the 

 same outline as the posterior edge of the fore-wing, so 

 that, when in use, the two fit pretty accurately together, 

 one behind the other. This alone, however, would not be 

 of much good, as the two wings would be easily liable to 

 be separated in flight, and thus break the force of the 

 stroke. But there is a beautiful arrangement which keeps 

 them in jjositiou relatively to one another, and thus enables 

 them to offer to the resistance of the air what is practically 

 an unbroken surface equal to their combined areas. 



The wings of a hornet, or failing these, of a humble 

 bee, or wasp, will show this arrangement admirably. For 

 a certain distance, beginning rather more than half-way 

 along the anterior edge of the hind-wing, there is a series 

 of booklets bent obliquely outwards, upwards, and back- 

 wards. Fig. 1 shows the arrangement of these in a 



Fig. 1. — A, Wings of Hornet, a, Fold on Fore-wing. 

 b, Hooks on Hind-wing. Magnified two diameters. 

 B, Portion of Hooked Margin. Mueh more liiglily 

 magnifled. 



hornet's wing. The number of booklets varies with the 

 insect ; for example, the hornet has 29, and the hive bee 

 about 23, bumble bees have from 19 to 26, according to 

 sjiecies and sex, and so on. In general the number 

 appears to range between one and two dozen, but the 

 ichneumon flies have fewer, while the giant Sirex, or 

 tailed wasp, the largest hymenopterous insect wo possess, 

 has upwards of !J0. 



Along the binder edge of the fore-wings, in the part 

 immediately opposite the books, the membrane is 

 strengthened and bent under, forming a sort of trough 



or gutter. When the wings are exi)anded, the booklets of 

 the hind-wing hang over the edge of this trough, and thus 

 the two wings are firmly linked together, so that any 

 movement of the one drags the other with it. And this 

 is advantageous in more ways than one. For in the Hymen- 

 optera, as in insects generally, the chief muscles of flight 

 are those attached to the fore-wings ; they are far more 

 bulky and powerful than tho.se belonging to the other 

 pair, hence the linkage com])ensates for the lack of 

 muscular force in the hind-wings, and enables them to 

 partake of the superior .strength of the primaries. Further, 

 it will be seen that the arrangement by which the books 

 of the hind-wings lay hold of a fold above them and under 

 the fore-wings, brings about a still closer union between 

 the wings during the down stroke when the greatest 

 power is required for jn-opelling the insect, whereas if they 

 were attached in the opposite way, there would be a 

 greater tendency for them to loosen their grip, and so 

 defeat their purpose. 



The books are simply modified hairs, and the hornet 

 shows a gradual change of form in them as we pass along 

 the series, from the completely booked and strongly grip- 

 ping clasper at one end, to the scarcely bent but stout 

 hair-like objects at the other, which have obviously very 

 little (■lasi)ing power left. The rest of the wing-margin 

 at both ends of the series is bordered by simple hairs of 

 the ordinary type. Of course these hooks, under the 

 most favourable circumstances, are extremely small, and 

 need the use of a microscope for their examination. But 

 their power is easily demonstrated in a fresh specimen of 

 the insect ; for if, by grasping the fore-wing between 

 thumb and finger or with forceps, we move it in any 

 direction, we find that the hind-wing of the same side 

 accom2)anies it in all its movements. 



In the Lepidof»tera, the order which contains the 

 butterflies and,- moths, the arrangement is of quite a 

 different character. In the butterflies, there is no special 

 apparatus to link the wings together, but their united 

 action in the down stroke is assured by the fact that the 

 fore- wing at its inner edge laps considerably over the hind 

 one. The depression of the one, therefore, carries the 

 other with it without difficulty. In moths there are two 

 different plans for bringing about the same result. Sup- 

 liose we take such a creature as the Ghost Moth {Hepialus 

 hnniuli). This is a very common insect, and is remarkable 

 as having a completely different coloration in the two sexes ; 

 the male is entirely white above and smoky beneath, while 

 the female is of a tawny yellow with reddish markings on 

 the fore-wings. Near the base of the fore-wing at its 

 inner edge in either sex, a small projecting lobe is seen, 

 of a membranous nature, but stiff, and furnished with a 

 fringe of long hair (Fig. 2). The jmints of attachment of 

 the two wings to the thorax in 

 this insect and its allies, are so 

 situated as to leave a small gap 

 at the base of the wings when 

 they are spread for flight. This 

 allows the above-mentioned 

 lobe to get beneath the margin 

 of the hind-wing, while the 

 rest of the fore-wing, as usual, 

 overlaps ; thus the wing is held, as it were, in a clip, and 

 the uniteil action of the two is aided to some extent, 

 though not very efficiently. 



This kind of link is found in two families of moths only, 

 viz., that of the Swifts, to which the Ghost Moth belongs, 

 and a family of extremely minute moths which rejoice in 

 the rather cumbrous name of Mirriijilcri/ijiil;)'. As these 

 minute insects are known ouly to the learned in such 

 matters, they have no popular name, and we must perforce 



FiG.2.— Ri] 

 Ghost Motb. 



;htForo-wingof 

 a, Jiigiim. 



