Usinger: Introduction 



Intro, fig. 8. Surface respiration by water bugs, a, fifth instar 

 nymph of Be/osfoma flumineum Say; b, fifth instar nymph of 

 Notonecta undulata Say; c, adult Ranatra fusca P. B. (Maloeuf, 

 1936). 



(3) tracheal gills that depend upon diffusion of dis- 

 solved oxygen from the water directly into the tracheal 

 system (many aquatic larvae, mayfly naiads, etc.), (4) 

 respiration (intro. fig. 7) by means of an air bubble from 

 which oxygen diffuses into the insect's spiracles and 

 into which oxygen diffuses from the surrounding water 

 (adult bugs and beetles), (5) direct contact with air 

 in plant tissues by inserting tubes into the roots and 

 stems of aquatic plants (beetle larvae of the genus 

 Donatio,, mosquito larvae of the genus Mansonia), 

 and (6) contact with atmospheric air by breaking the 

 surface with hydrofuge hairs or surfaces (intro. fig. 

 8a, b) (adult beetles and bugs) or with breathing tubes 

 (intro. fig. 8c) (water scorpions, mosquito larvae, etc). 

 Of all these, the most remarkable is the silvery 

 bubble of adult beetles and bugs that serves as a gill, 

 holding approximately 80 percent N and 20 per cent 

 2 when first formed at the surface. When the insect 

 submerges, the oxygen in the bubble begins to de- 

 crease as it is used up, thus lowering the volume of 

 the bubble and reducing the ratio of 2 to N. To com- 

 pensate for this lower oxygen tension, oxygen diffuses 

 into the bubble from the surrounding water, and since 

 the invasion coefficient of oxygen between water and 

 air is three times as great as that of nitrogen, the 

 insect is able to remain submerged much longer 

 (thirteen times as long in one experiment) than if it 

 were dependent on surface oxygen alone (Comstock, 

 1887). Theoretically it is only when all the nitrogen 

 has diffused outward that the system breaks down 

 and new surface air is required. 



Intro, fig. 9. Diagrams to illustrate the wetting of: o, a system 

 of short, stiff, erect hairs; and b, c, a system of longer hairs bent 

 to form a more or less horizontal and compressible mat (Thorpe 

 and Crisp, 1947). 



The bubble of changing volume is held by hydrofuge 

 hairs which lie more or less parallel to the body 

 surface in a compressible mat. The system is illus- 

 trated (intro. fig. 9b and c), showing the angle of 

 contact, of water on the waxy surfaces of the hairs 

 (Thorpe and Crisp, 1947). 



A few beetles (Dryopidae) and an old-world water 

 bug (Aphelocheirus) have a plastron of fixed volume, 

 maintained by stiff hairs of a density up to 2 million 

 per square millimeter (intro. fig. 9a). Unlike the larger 

 bubbles described above, the plastron is virtually 

 incompressible and hence can act as a permanent gill 

 or avenue of diffusion of oxygen from the water to 

 the tracheal system. These are among the very few 

 permanently aquatic insects that do not need to come 

 to the surface at any time during their life cycle 

 (Thorpe, 1950). 



Osmoregulation. — The regulation of osmotic pres- 

 sure of body fluids is another important type of 

 adaptation in aquatic insects. Many marine animals 

 have body fluids that are isotonic with sea water. 

 All fresh-water organisms have some method of regu- 

 lating the concentration of their body fluids. In the 



Intro, fig. 10. Terminal segments of Culex pipiens L. larvae 

 showing typical appearance of anal papillae when reared in media 

 of increasing salt concentration, a, larva reared in distilled 

 water with mean length of papillae 0.82 mm; b, tap water (0.006 

 per cent NaCI) — 0.36 mm.; c, medium with 0.075 per cent NaCI — 

 0.33 mm.; d, medium with 0.34 per cent NaCI— 0.22 mm.; e, 

 medium with 0.65 per cent NaCI — 0.20 mm.; f, medium with 0.90 

 per cent NaCI— 0.20 mm. (Wigglesworth, 1938). 



