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Sizing Up the Hexapods

Session leader: Dave Bellamy.

Table of Contents

  1. What is the range of hexapod size?
  2. What factors limit hexapod size?
    1. Upper limits
    2. Lower Limits
  3. What is the evolutionary trend for hexapod size?
  4. Is size the reason for hexapod abundance?
  5. References

Four orders of magnitude separate the length of a springtail, family Isotomidae (.5 mm) and a Hercules beetle, Dynastes hercules (151 mm). Furthermore, there are six orders of magnitude that separate their mass (25 micrograms vs. 75 grams, respectively). Amazingly, these two do not even represent the hexapod extremes.

Springtail picture from Iowa State University and Hercules beetle picture from the Smithsonian Institute

What is the range of hexapod size?

Hexapod size varies more than in any other class of animals, with length variations covering four orders of magnitude, and mass variations of seven orders of magnitude for extant hexapods. It is important to note that the "range" discussed in this section has nothing to do with "limits" in body size. Just because we know of no hexapod to ever have been larger than 750 mm does not necessarily mean that hexapods cannot be larger. The same holds true for the smaller hexapods. Possible limiting factors will be discussed in the following sections and during the class discussion.

Some of the smallest hexapods are members of springtails (Neelidae) (0.27 mm), thrips (0.5 mm), minute bog beetles (Sphaeriidae) (0.5 mm), feather-winged beetles (Ptiliidae) (0.25 mm), and many Chalcidoidea (Hymenoptera) that include the smallest members, fairyflies (Mymaridae) (<< 0.5 mm) and Trichogrammatidae (0.18 mm). The Trichogrammatidae are parasites of thrip eggs and may have a mass less than one microgram!

Some of the largest insects only flourished during the Carboniferous period (350 mya). This was a period dominated by many large dragonfly-like insects, including Meganeuropsis permiana Carpenter, within the extinct order Meganisoptera. This monster had a wingspan between 710 mm and 750 mm (or about 28 inches), making it the largest known insect ever to have existed. Click here to see photos of other fossil dragonflies.

Some of the largest extant hexapods include dragonflies (Aeshnidae)(135 mm), Phasmida (310 mm), the Chinese mantid (Montodea)(>100 mm), some tropical species of queen termites (Isoptera)(110 mm), giant water bugs (Belostomatidae)(>100 mm), a tropical plant hopper (Fulgoridae)(150 mm wingspread), dobsonflies (Corydalidae)(130 mm wingspread), and a number of beetles (Dynastinae, Cetoniinae, Cerambycidae, Buprestidae, etc.)(>100 mm). Click here to see photos of some of the beetles mentioned, including the Goliath beetle with a mass of 100 grams!

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What factors limit hexapod size?

Now that the range has been examined, we can speculate as to whether this range is actually the limit for the hexapod body plan. Is this limit a physical one or an environmental one? Both categorical limitations could exist, but surely one is encountered before the other. Which one, then, has more of an impact on hexapods? What factors might actually account for the range in size and mass that we see? Have these factors always existed, or are they newly imposed? Are these factors universal within the animal kingdom, or are they unique to the insect world? Because of the scarcity of definitive literature on this topic, the answers to these questions currently remain speculative. Here are some ideas that have been put forth within the literature (and some through discussions).

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What is the evolutionary trend for hexapod size?

The theory that animal body sizes change over time has been examined extensively (for vertebrates!). The apparent tendency for body size to increase over time in many diversifying taxonomic groups has been referred to as Cope's Rule (Cope, 1887). Explanations for this trend have usually centered around specific advantages of large individuals, such as ability to tolerate short-term variation in the physical environment, capacity to extract energy and nutrients from a wider variety of poorer quality food, and ability to avoid many kinds of predators. Cope's Rule has usually been used in the discussion of birds and mammals. What trends do we see in the hexapod world? Does Cope's Rule hold for insects as well? If not, why not?

Many entomologists might say that hexapods have been getting smaller. Perhaps this is due to the brief presence of the giant Protodonates during the Carboniferous period. It would be easy to draw a conclusion from the fossil evidence that dragonflies have become smaller over time. Indeed, many of the largest insects known to exist flourished during the Carboniferous only to become extinct. However, the presence of giant insects does not exclude the presence of small insects. Actually, the oldest known fossil hexapod is a springtail (Rhyniella praecursor Hirst & Maulik) from the lower Devonian. Scourfield (1940) judged from the length of the head, that the specimen appears to have been from 0.3 to 0.4 mm, a size that corresponds closely with those Collembolans living today. In fact many fossil records show that many hexapods have remained consistent in size throughout their existence. Not only did many Carboniferous giants become extinct, but so did their orders. Therefore, one cannot argue that these orders have become smaller over time, they simply were unsuccessful as insects and died out, perhaps allowing for future increases in hexapod diversity.

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Is size the reason for hexapod abundance?

All this speculation on factors that limit size may be important when addressing the issue of insect diversity. Hexapods may be the most diverse and numerous -- both in terms of species richness and individual population -- life form on Earth. How have they managed to succeed so well? It has been suggested in many introductory biology textbooks that the ability to diversify depends on the homogeneity of the environment. The larger an animal is, the more homogenous their environment appears in terms of available niches. Smaller animals, on the other hand, live in environments rich with possible niches. However, this does not mean it's better to get smaller. As we have seen, there are many influential limiting factors present in the environment.

Siemann et al (1996), examined species diversity, abundance and body size relationships in an grassland insect community and found that the intermediate body sizes for each sampled order led to a maximum individual population that occurred during peak species richness. There appears to be an optimum body size for each order sampled. While this information is interesting and might explain the abundance of hexapods, it does not address the factors that actually limit size.

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Required papers

Full references list

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Nanosella photo copyright © 1997 W. Eugene Hall

Page copyright 1998, David Bellamy.