Introduction to Aquatic Entomology 



A. Principles and Practices 



By Robert L. Usinger 



University of California, Berkeley 



Insects are generally the most conspicuous forms of 

 life in ponds and streams and occur in tremendous 

 numbers in such unlikely places as the bottoms of 

 lakes. No other group of animals shows such diver- 

 sity in structure and habits. And yet, aquatic insects 

 fall into a pattern, each major group (order, family, 

 or genus) occupying a particular habitat with species 

 represented on each of the continents. Thus a stream 

 in South Africa may resemble a stream in California, 

 each having its own representatives from among the 

 stoneflies, mayflies, caddisflies, and so on. Like- 

 wise, a pond in Sumatra and a lake in Sweden will 

 resemble, in a general way, comparable bodies of 

 water in North America. This same phenomenon is 

 noted among the common genera of plankton organisms. 

 However, many of the plankton species are cosmopol- 

 itan whereas aquatic insect species are usually limited 

 in their distribution, with some species restricted to 

 local oases in otherwise completely barren deserts. 

 This leads to a multiplicity of insect species and, of 

 course, provides the basis for still greater diversity. 

 It is thought that aquatic insects were derived from 

 unknown terrestrial types which invaded the water on 

 several occasions during the course of their evolution. 

 The first record is of mayflylike insects of the now 

 extinct order Paleodictyoptera. These appear suddenly 

 in the geological record in rocks of the Upper Carbon- 

 iferous period. At this remote time, 250 million years 

 ago, the first winged insects had complete mastery 

 of the air because no bird, bat, or flying reptile had 

 yet developed to challenge them. The subsequent 

 history of aquatic insects (and terrestrial forms, as 

 well) was one of increasing diversity. First a wing- 

 folding device rendered all higher insects (Neoptera) 

 more efficient in flight than the dragonfly and mayfly 

 types (Paleoptera). Second, the direct method of 

 development through successive instars with external 

 wing pads (stoneflies, true bugs) was improved by 

 the adoption of a more indirect series of stages with 

 larvae specialized for feeding, pupae for transforma- 

 tion, and adults for reproduction. Wing pads develop 

 internally in such larvae. These advances came before 



the end of the Paleozoic era (200 million years), and 

 all subsequent evolution was confined to modifications 

 of these basic patterns of structure and development. 



At the end of the Paleozoic and during the Mesozoic 

 and Cenozoic eras representatives of many modern 

 groups of insects took to the water. The exact se- 

 quence is not known, but probably the stoneflies were 

 among the earliest with dobsonflies, beetles, and true 

 bugs not far behind (Permian, 220 million years). 

 Caddisflies, true flies, and parasitic wasps fir<t 

 appear in the very fragmentary record much later 

 (Jurassic, 160 million years), and the Lepidoptera 

 not until early Tertiary (60 million years). (See Carpen- 

 ter, 1953, for a summary of information on the geological 

 history of insects.) Furthermore, it seems certain that 

 each of the large orders (Colooptera, Diptora, etc.) 

 invaded the water not once but several times. As Mi all 

 (1895) puts it, "I think we can say with a considerable 

 degree of probability that this change of habitat from 

 terrestrial to aquatic has taken place in the class of 

 insects at least a hundred times quite independently, 

 and the number may be very much higher than a hun- 

 dred." As a result we have a most amazing variety 

 of insects occupying aquatic habitats, many with a 

 superficial similarity in form owing to the highly 

 selective environment but each group with unique 

 methods for performing essential life functions. 



At present, ten orders of insects have truly aquatic 

 forms, and several others may be described as semi- 

 aquatic, at least in part. All these except the beetles 

 and true bugs live on land or in the air as adults and 

 in the water only in their immature stages. In contrast 

 to this, most water bugs and beetles are aquatic 

 throughout their lives but are directk dependent on 

 surface air for respiration as adults. Thus it can be 

 said that despite their great numbers and remarkable 

 diversity, insects are only secondarily and incom- 

 pletely adapted to aquatic life. This probabh accounts 

 for their prevalence in shallow ponds and streams, 

 their scarcity in very large rivers and deep lakes, 

 and their virtual absence from the open waters of 

 the ocean. 



