. Entomology : with special reference to its biological and economic aspects. he center of gravity, are of most use in swim-ming, though the second pair also are used for this purpose;indeed, a terrestrial insect, finding itself in the water, instinc-tively relies upon the third pair of legs for locomotion. Hy-drophilus uses its oar-like legs alternately, in much the samesequence as land insects, but Cybistcr^ other Dytiscidse,which are even better adapted than Hydrophilus for aquaticlocomotion, move the hind legs simultaneously, and therefore?can swim in a straight line, without the woljb
. Entomology : with special reference to its biological and economic aspects. he center of gravity, are of most use in swim-ming, though the second pair also are used for this purpose;indeed, a terrestrial insect, finding itself in the water, instinc-tively relies upon the third pair of legs for locomotion. Hy-drophilus uses its oar-like legs alternately, in much the samesequence as land insects, but Cybistcr^ other Dytiscidse,which are even better adapted than Hydrophilus for aquaticlocomotion, move the hind legs simultaneously, and therefore?can swim in a straight line, without the woljbling and lesseconomical movements that characterize Hydrophilus. ADAPTATIONS OF AQUATIC INSECTS 187 Larvae of mosquitoes propel themselves by means of lash-ing, or undulatory, movements of the abdomen. A peculiarmode of locomotion is, found in dragon fly nymphs, whichproject themselves l:)y forcibly ejecting a stream of water fromthe anus. On account of the large auKiunt of air that they carry about,most aquatic imagines are lighter than the water in which they Fig. Left hind legs of aquatic beetles. A, Hydrot^IiilKS triangularis; B, Cybistcr fimbria-latiis; c, coxa; /, femur; j;, spur; t. tarsus; ii, tibia; tr, trochanter. live, and therefore can rise without effort, but can descend onlyby exertion, and can remain below only by clinging to chancestationary objects. The mosquito larva ( Fig. 229, A) is oftenheavier than water, but the pupa (Fig. 229, B) is lighter, andremains clinging to the surface film. The tension of this surface him is sufticient to sui)port theweight of an insect up to a certain limit, provided the insect i88 ENTOMOLOGY Fig. 22g. has some means of keeping its body dry. This is accom-pHshed usually by hairs, set together so thickly that water cannot penetrate between the legs and body of Gcrrisare rendered water-proof by a vel-vety clothing of hairs, the insect,though hea\ier than water, is ableto skate about on the , by means o
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Keywords: ., bookcentury1900, bookdecade1900, booksubjectentomology, bookyear1
