The size of an insect individual is determined by its genes and by the environment in which it grows. Temperature, crowding, food quantity, and food quality are examples of environmental factors that affect size, but insects may make up for such effects by compensatory feeding.
The size of female insects often determines their fecundity, which may be manifested in giving birth to many small offspring or a smaller number of large ones. To be able to produce many large offspring, which may be adaptive for survival in a harsh environment, the female herself must be large. The importance of size for female fecundity can often be seen in the sexual dimorphism of insect species, in which males typically are much smaller than females.
Although the primary role of male insects is to fertilize the eggs, males may benefit from being large because they contribute to the realized fecundity of females by providing resources through their ejaculate or in competing with other males to obtain mates. An example of the latter characteristic is provided by some species of digger wasps (Sphecidae), in which males compete intensely with each other for females only half their size.
Sometimes, adult or larval foods come in packages or shapes that allow only very small insects to use them. Such foods include very small items like seeds and insect eggs, or very thin items like pores of fungi. Many insect families that use these foods [e.g., bruchid beetles, mymarid parasitic wasps, and nanosellin (Ptiliidae) beetles, respectively] have been adapted to and have radiated into several species under such living conditions.
Insects smaller than 1 mm operate in a world where gravity and molecular forces are in the same order of magnitude. This can be advantageous when, for example, insects find it easier to climb vertical surfaces. However, it can also lead to problems when, for example, an insect is trapped in a drop of water by the water's surface tension.
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