April 26, 2018
It’s a bug hunt…
Today is Alien Day, a day to appreciate the Sci-Fi horrow/action Alien franchise! The date is not entirely random – today is April 26 (4-26), and in the first two films of the franchise, the titular creatures were encountered on the planetoid LV-426. While the creatures of the film are fictional, many aspects of their biology are shared with one or more species of insect. So with this blog post, let’s nerd out about the science non-fiction that parallels the science fiction and see just how alien some of our pests really are.
First, let’s recap what the alien (named a xenomorph in “Aliens”) is and highlight some of the features that it shares in common with real-life animals. The xenomorphs are social creatures, forming hives from secreted resins that contain reproductive queens and sterile workers that build the hive, gather food, etc. They have a parasitic life cycle. Eggs hatch into small spidery facehuggers that attach to a host, incapacitate it, and implant it with a parasitoid life stage. The parasitoid develops internally, and upon reaching maturity it bursts out of the host’s chest, killing the host in the process. All life stages are protected by a tough exoskeleton and corrosive blood. Adults have an extensible inner mouth that they can extend like a tongue from their normal-sized jaws. When foraging, adults haul hosts back to the nest alive and cocoon them where the facehuggers can easily access them. No real species shares all of these characteristics, but a variety of species each exhibit one or more of them. So without further ado, let’s hit some examples!
Social structure: Ants, some bees, some wasps, termites, and a few other insects share similar social structures to that exhibited by the xenomorphs. The queen, with her gigantic distended brood sac, is particularly reminiscent of the tremendously bloated queens of termites, though the less intricate social structure is more typical of bumble bees or yellowjackets.
Nest structure: The resinous hive walls look similar to those produced by some species of stingless bees, which use plant resins to build a sort of comb[i]. They also resemble the wax nests of honey bees and bumble bees or the paper nests of yellowjackets, hornets, and paper wasps. However, the xenomorphs seem to make their hives entirely out of bodily secretions, so the webspinners (order Embioptera) might be a closer analogy. These small insects live in communal tubes that they rapidly construct from silk secreted by their front feet.
The parasitic life-cycle: A life cycle in which an animal feeds on a single host and ultimately kills it or castrates it is known as a parasitoid. Insect parasitoids take many forms. Perhaps the best known are wasps, which usually tightly specialize on one or a few closely related hosts. The female wasp generally injects her eggs directly into or onto her host by stinging it. The next largest group of parasitoids are included within the flies. Often overpowering their host’s immune response by sheer toughness and a knack for gnawing breathing holes out of encapsulating immune cells, flies are often less host-specific than wasps. They also use a greater variety of tactics to get their eggs or larvae into or onto a host, with some injecting eggs like wasps and others dumping eggs in promising locations and letting the maggots do the hunting, much like the xenomorph facehugger. Several types of parasitoid rove, wedge-shaped, and blister beetles and the very odd twisted wing parasites also take the mobile infective young larva approach[ii]. Most parasitoids are beneficial, and they can be the dominant natural population controls on their host insects. One exception is the cluster fly, which parasitizes earthworms through the summer and invades buildings in the fall to overwinter.
Armor-like exoskeleton: All arthropods have exoskeletons, but not all are equally tough. With their specialized shell-like front wings, the beetles probably include the most heavily armored of insects. In particular, ironclad beetles are known to be particularly troublesome to a collecting entomologist because an insect pin will likely bend or slide across the shell rather than puncture this insect, and many collectors pre-drill them before placing a pin[iii]. More likely to be found around structures, the weevils are also remarkably heavily armored. This family of beetles, which includes some turf and ornamental pests, sporadic home invaders, and pantry pests, have such tough exoskeletons and can hold their breath for long enough that I have often had them scurrying around healthily in a kill jar loaded with ethyl acetate long after all other insects have perished.
Corrosive blood: The Asian multicolored lady beetle is the first pest insect that comes to my mind with defensive hemolymph (the insect version of blood). Their yellow hemolymph is loaded with pungent bitter alkaloids, and the beetle can reflexively bleed from its leg joints if it is disturbed[iv]. However, some beetles have far more potent chemical defenses. Some species of rove beetles in the genus Paederus have an extremely potent skin irritant in their blood that causes painful blistering on contact with skin. In specific circumstances, these beetles may occur in huge numbers and swarm, sometimes landing on people. If a toxic Paederus beetle is crushed as it is brushed off, its smeared hemolymph will leave a trail of itching, burning, blistering rashes. Blister beetles are comparably toxic, but are large enough and sedentary enough to rarely end up on people[v]. However, some species of blister beetles feed on alfalfa, and blister beetle contamination in hay has been known to kill horses that eat the contaminated hay[vi].
Extensible mouth: Two types of insects have mouthparts that can launch forward from the head to strike at prey; immature members of the order Odonata (dragonflies and damselflies) and rove beetles in the genus Stenus[vii]. In both cases, the structure is a modified labium, the insect version of a lower lip. These projectile lips are tipped with hooks that allow them to yank prey back into the mandibles to be properly chewed up and eaten.
Host-hunting adults: The spider wasps of the family Pompilidae may be the best example here, including the huge and intimidating tarantula hawk wasps. These wasps, which are often medium-sized to very large and commonly dark metallic blue-black, seek out spiders and paralyze them with a sting, then haul them back to their burrows, where each spider will nourish a single wasp larvae from egg to adulthood. A slightly more conspicuous host-hunting wasp is the cicada killer. Comparable in size to a large hornet, these wasps may be noticed when males are defending territories, females are hauling paralyzed cicadas, or females are digging burrows. The males are territorial to anything that invades their territories, and will buzz around the face of intruders menacingly. Fortunately, the sting is part of the female reproductive tract, meaning these territorial males lack the hardware to forcefully defend their territories against anything considerably larger than themselves. Females can sting, but generally prefer to fly away over stinging.
References:
[i] Roubik, D.W., 2006. Stingless bee nesting biology. Apidologie, 37(2), pp.124-143.
[ii] Eggleton, P. and Belshaw, R., 1992. Insect parasitoids: an evolutionary overview. Phil. Trans. R. Soc. Lond. B, 337(1279), pp.1-20.
[iii] Le Nguyen, V., 2017. Structure-Property Relations of the Exoskeleton of the Ironclad Beetle (Zopherus nodulosus haldemani) (Doctoral dissertation, Mississippi State University).
[iv] Sloggett, J.J., Magro, A., Verheggen, F.J., Hemptinne, J.L., Hutchison, W.D. and Riddick, E.W., 2011. The chemical ecology of Harmonia axyridis. BioControl, 56(4), pp.643-661.
[v] Ghoneim, K.S., 2013. Human dermatosis caused by vesicating beetle products (Insecta), cantharidin and paederin: An overview. World J Med Med Sci, 1(1), pp.1-26.
[vi] Schmitz, D.G., 1989. Cantharidin toxicosis in horses. Journal of veterinary internal medicine, 3(4), pp.208-215.
[vii] Koerner, L., Gorb, S.N. and Betz, O., 2012. Functional morphology and adhesive performance of the stick-capture apparatus of the rove beetles Stenus spp.(Coleoptera, Staphylinidae). Zoology, 115(2), pp.117-127.
