Because of its structure, the environment offers many more niches for small organisms than for large ones. The relatively small size of insects, which is one of the reasons for their success, has therefore made them very diverse, a characteristic that has resulted in a high number of species. In spite of their generally small size, limited by their method of gas exchange, insects show as large a range in size as other groups of organisms.
SIZE VARIATION
Size varies tremendously among and within orders, families, and species of insects. The smallest extant insects known are about 0.2 mm in length and can be found among beetles of the family Ptiliidae and wasps of the family Mymaridae, which are egg parasitoids. Insects of this minute size are smaller than the largest one-celled protozoans. The largest extant insects are phasmid (walk-ingstick) species (up to 30 cm long), sphingid moths (wingspan of up to 30 cm), and some beetles of the genera Megasoma, Dynastes, or Goliathus (up to 100 g).
Although insects are not as large as some other organisms, the range in size among insects is almost as large. For example, the difference in volume between the largest mammal (the blue whale) and the smallest (a minute shrew) is about 1:2 X 108. This is comparable to 1:1.5 X 108 for the two extremes among insects (ptiliid beetle and Goliath beetle).
SOME EXPLANATIONS FOR VARIATIONS IN SIZE
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 question of why a given insect (or for that matter any other organism) is of a particular size ultimately requires an evolutionary answer. In principal, the size of an insect is determined by the fitness benefits of a larger size (e.g., greater reproductive output) and the fitness costs of reaching this larger size (e.g., higher larval mortality). Natural and sexual selection should favor an optimal body size where the fitness costs of further growth become higher than the expected fitness benefits of this extra growth. Obviously the nature of these costs and benefits depends on the phylogenetic history, genetic composition, and ecological situation of the species in question.
The size of female insects often determines their potential fecundity, but to what degree an increase in this potential may be realized in the field is dependent on weather conditions, availability of hosts, and abundance of natural enemies that influence the expected life span. The nature of such limiting factors will influence how strong the benefits of a larger size are likely to be and therefore also affect optimal size.
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. As the reproductive tactics of males and females differ, the sexes also differ in how size affects reproductive success and, hence, optimal size. In fact, sexual size dimorphism is particularly instructive in explaining variation in optimal size as it shows that differences in selective regimes may lead to adaptive size differences also within a given species.
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 (Fig. 1 ).
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.
FIGURE 1 Sexual dimorphism in insect body size illustrated by a species of digger wasps (Sphecidae) in Chile. The male is larger than the female most probably as a consequence of strong competition among males.
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.
FACTORS THAT LIMIT SIZE
The smallest insects will have difficulty making room for the internal organs that are necessary for their existence. For example, some ptiliid beetles can lay only one egg at a time because their eggs may be up to 0.7 times the size of the whole insect.
The largest size an insect can reach is limited by the tracheal system. In insects, gas exchange with air is mediated directly to the tissues by a highly branched system of chitin-lined tubes called tracheae. No cells in the insect body are more than 2-3 |im from a tra-cheole. Diffusion along a concentration gradient can supply enough oxygen for small insects, but forms that weigh more than about a gram, or are highly active, require some degree of ventilation. Most insects have ventilating mechanisms to move air in and out through the tracheal system, but the need to allow enough oxygen to reach the tissue by diffusion imposes limits on tracheal length. Most large insects present today have long slender bodies, a trait that also limits tracheal length. Furthermore, elaborations of the tracheal system could not be made without destroying the water balance in large insects. However, there are exceptions: some of the heaviest extant beetles have bulky bodies, but these insects are not (or do not have to be) very quick and do not fly.
On a smaller scale, it is often difficult to understand why many insects do not evolve toward larger sizes. The reason behind this paradox is that insect larvae normally have a great capacity for rapid weight gain, and relatively small changes in development time may lead to very substantial increases in final size. In the tobacco horn-worm (Manduca sexta), which is a Sphingid moth, 30 years of laboratory evolution lead to a 50% increase in pupal size due to small differences in growth trajectory. If all this extra mass could be realized as increased fecundity, it is difficult to see why natural selection in the wild did not favor this size increase. It has therefore been suggested that field conditions often limit the degree to which increases in potential fecundity (mass) can be successfully realized into real reproductive success.
CHANGE OVER TIME
It has been suggested that organisms increase in size over an evolutionary time scale. However, there is no evidence to support this suggestion, and perhaps natural selection acts on correlated traits that constrain the evolution of increased size. In fact, fossils reveal that some insects in the past were much larger than their extant relatives. For example, many winged Carboniferous and Permian insects, existing about 300 mya, had wingspans exceeding 45 cm; the largest was the Permian dragonfly Meganeuropsis schusteri, which had a wingspan of 71 cm. These insects certainly also had long, narrow bodies, to reduce the length of the trachea. During these prehistoric times, the atmospheric oxygen concentration was much higher (up to about 35%) than the present level (20.9%), which may have allowed sufficient oxygen to reach the innermost tissues of very large insects. However, such an oxygen-rich atmosphere also would have augmented aerodynamic properties in early flying insects. It has been suggested that later-appearing insects could not evolve to a large size because of competition for niches with birds and other later-appearing animals.
OTHER RELATIONSHIPS
Latitudinal and altitudinal clines in body size have been of interest for biologists for a long time. In insects there are examples where body size within a taxonomic group increases with latitude (Bergmann clines), but there are examples where size decreases (converse Bergmann clines) or stays relatively constant with latitude. These patterns often have a genetic basis, but there is a very common pattern also where insects grow phenotypically larger when reared in colder conditions. In general, it is still not clear to what degree the documented size clines are the result of differential adaptation to clinal variation in temperature and season length, or of constraints on how temperature affects growth and development in ectotherms.
Insect assemblages are thought to be structured by competition, with most of the insects found in medium-sized classes. Thus, the size of a particular insect is governed by its living habits and its feeding guild, in which competition with similar insects has forced some to evolve a larger or smaller body size. Empirical data show that species diversity in any taxonomic group of insects peaks at some intermediate body size. More knowledge about the causes behind size distribution patterns among insects and other organisms may provide key information in the effort to preserve biodiversity.
