I often get asked silly questions, and the most frequent is the one about which animal group is the most successful. Beetles (Coleoptera). They’re found everywhere except in the sea and there are over 300000 described species (estimates as to how many there are go up to 8 million species).
The point of this post is to examine the reasons for their success. As insects, they already have some considerable advantages: their small size allows them to evade predators and disperse easily, both of which are helped by their ability to fly. Their reproductive strategies are also helpful for evolutionary success: short life history + large number of eggs = greatest variation for natural selection to act on, meaning they can quickly adapt to new environments. The development of a larva-pupa-imago system is also key to conquering new habitats: the larvae collect food stores for when they turn into an adult; the adult is then only concerned with dispersal and reproduction. In many species, the pupa is a perfect stage to withstand harsh conditions.
These are all features of insects in general. But within the insects, beetles are the most diverse – and thus successful – order. Let’s look at what makes them different from the others.
The picture above [Original here!] shows the (arguably) most important feature of the beetles: the modification of the first wing pair to a hardened elytra. When not flying, the hindwings fold (often along the veins) under the elytra as a protective measure. The elytra also covers up the spiracles, which reduces water loss.
The picture above [Source!] shows another characteristic feature of the beetles: they are very compact. They achieve this by housing the coxa (first leg segment) in a cavity (see below).
Both these features may seem discrete, but they lie behind the staggering success of the beetles, by allowing them to occupy all sorts of habitats, from small chinks in a wall (think of cockroaches) to deserts. With such a high colonisation ability, it’s no wonder that they are so dominant in most terrestrial ecosystems.
As for the origin of this beetle supremacy (melodrama!), it is linked to the evolution of their first feeding source, land plants, although their great radiation paralleled that of the flowering plants (Farrell, 1998), as part of the insect – angiosperm coevolution.
But let’s not forget that at the core, “Which animal group is the most successful?” is a ridiculous question. Success has no value; the criteria you set for it can be wildly different. I based it on species number. Others would base it on sociality level, in which case the ants would win. Others might set it based on intelligence, in which case the octopi (or the humans) would win. I am always tempted to answer by naming the sponges: no matter how complex and elaborate other animals became, they remained more or less the same over the past 600 million years. Survival time is, to me, the Acid test for any organism group.
But beetles are sexy, so they win out anyway.