Adaptations of Aquatic Insects 



All living organisms are variously adapted for survival 

 in their respective environments. Many adaptations 

 are so commonplace that they are taken for granted. 

 However, the requirements for existence in aquatic 

 habitats are so rigorous that the adaptations are 

 usually striking. The stream-lined form, as seen in 

 certain fishes and mayfly naiads of swift-flowing 

 streams, and the flattened body with suction discs 

 seen in Psephenid larvae (waterpennies) and certain 

 fly larvae (Blepharoceratidae, Deuterophlebiidae, 

 Maruina) that cling to surfaces in rapids, are exam- 

 ples. Other common adaptations, especially among 

 adult aquatics, are reduction in size of antennae 

 which are concealed to reduce water resistance, 

 development of powerful legs with swimming hairs, 

 and presence of hydrofuge hairs or waxy surfaces to 

 prevent wetting. The latter are particularly important 

 at critical periods such as time of hatching of the 

 egg and time of emergence of the adult. Without such 

 adaptations the emergence of a delicate simuliid fly 

 from its pupal case attached to a rock in swiftrflowing 

 water would be impossible. 



Surface film. — The nature of the surface film is of 

 greatest importance to aquatic insects because of 

 their amphibious existence. To an organism of small 

 size this air-water interface can be an impenetrable 

 barrier, a surface on which to rest, or a ceiling from 

 which to hang suspended. At the surface the water 

 molecules are arranged in such a way that a surface 

 tension is created. This can be demonstrated by a drop 

 of water on a waxy surface (intro. fig. 4). The angle 



Intro, fig. 4. Diogrom to show the angle of contact, made by 

 a drop of fluid on a waxy surface, where yS is the solid-air 

 tension, yLS the liquid-solid tension, and yL the liquid-air ten- 

 sion (Thorpe and Crisp, 1947). 



of contact d under these circumstances is 105°- 

 110°. Addition of soap or some other wetting agent to 

 the water changes the angle of contact so that the 

 bubble spreads across the wax surface. 



/"k. 



3^-0= 



Intro, fig. 5. Diagram showing positive and negative menisci 

 with respect to various emergent and floating objects. Stems 

 that are wettable pull the surface about them into upward 

 slopes, or positive menisci, and stems that do not wet readily 

 (with waxy surfaces) bend it downward into negative menisci 

 (Renn, 1943). 



Usinger: Introduction 



In nature the an^lo of contact USUall) > in a 



negative meniscus when the water surface is In con* 



tact with the waxy surla< sen plan; 



or loaves extending above the water (intro. 

 This soon changes to a positive meniscus, however, 

 owing to the accumulation of wettable gelatinous 

 materials or to the death of the plants and merit 



loss of wax at die b ur faces, rhe lino of intersection 

 betwoen the three interface-, water-air, water-plant, 

 and plant-air, has been termed the "Intersection Line" 

 by Hess and Hall ( 1945), and the number of meter 

 intersection line per square meter of water surface is 

 called the "Intersection Value." 



Insects are variously adapted u> the intersection 

 line or meniscus. Anopheles mosquito larvae, for 

 example, are drawn tail first toward a negative me ulc- 

 eus from a distance of 9 mm. by force- independent of 

 their own efforts (intro. fig. 6) (Renn, 104:5); the larvae 

 of Dixa midges spend most of the time in positive 

 menisci where the water surface moots a wettable 

 surface such as a stone. Other insects, such as water 

 stridors, are adapted to life on the surface film where 

 their hydrofuge (non-wettable) tarsi bend but do not 

 break the surface. 



A B 



Intro, fig. 6. Diagram showing the pull of positive menisci at 

 a wettable surface (A) on the upward-bent tail of a model Ano- 

 pheles and the reverse action on the downward-bent head. The 

 effects with respect to non-wettable surfaces are shown in B. 

 The pull extends for a distance of 9 mm. (Renn, 1943). 



Aquatic respiration. — Possibly because of their 

 origin as terrestrial air breathers, insects have devel- 

 oped the most remarkable adaptations for aquatic 

 respiration. These include: (1) blood gills with hemo- 

 globin (chironomid larvae or bloodworms), (2) cuticular 

 respiration by simple diffusion into the tracheal 

 system (immature stages of most aquatic insects), 



, , . 1 1 .n i 



Intro, fig. 7. a, Dryops freshly submerged, crawling along stem 

 enclosed in its bubble; D-d, Ochthebius, dorsal, ventral, and 

 lateral views of submerged insect. The extent of the air film is 

 indicated by dotted lines in b and c, and by stippling in d. Wetted 

 areas are black (Thorpe, 1950, in port after Hase). 



