The ultimate couch potatoes

A fully engorged female dog tick (Dermacentron variabilis) is nearly completely incapable of movement
[Canon 7D, Canon 100mm macro, 3 speedlights Canon 580EXII]

The one big difference between terrestrial and aquatic environments is that it is really difficult to be a sedentary animal on land. Under water, with a constant stream of plankton and organic debris floating around, it is just so much easier to sit down, open your mouth, and let things just fall in. Entire phyla of animals do just that in oceans and lakes, but in dry environments few animals have achieved perfection of this type of behavior. Orb weaving spiders come close – they sit motionlessly and let insects fly into the web, but they still need to do the killing manually, so to speak. Other sit-and-wait predators fall under the same category. But a few groups of insects have become so truly sessile that they have begun to resemble marine organisms physicaly. And by sessile I mean truly immobile, with no ability to move on their own, even if their life depended on it.

Tick-fly (Forcipomyia tettigonaris) attached to the wing of a katydid (Cycloptera speculata)(Suriname) [Canon 7D, Canon 100mm macro/Canon MP-E 65mm, 3 speedlights Canon 580EXII]

The most important problem for terrestrial animals who would love to become land versions of sponges or sea anemones is that air is much thinner than water, and thus less likely to carry a lot of nutritional particles. And even if it did, non-organic elements, such as dust and sand would probably quickly outweigh the food and cause physical damage to anybody trying to filter out the good stuff. For this reason the only option available to aspiring terrestrial polyps is to get a steady supply of nutrients by tapping into a water-based source. And there is no better such a source than other organisms’ blood, or its equivalent.

Tick flies (Forcipomyia ixodoides group) permanently attached to the antenna of a walking stick (Costa Rica) [Canon 7D, Canon MP-E 65mm, 3 speedlights Canon 580EXII]

Three groups of arthropods turn into completely immobile parasites during at least on stage of their life, but each of them arrived at this particular juncture of the ultimate laziness through very different paths. The most familiar, but also not fully committed to the sedentary life, are mites. Ticks (Ixodida) are a group of mites that feed on the blood of vertebrates, and the last stage of a female’s life is spent in the form of a huge, blood- and egg-filled sack with legs that are too short to do her any good. Larvae of some velvet mites (Parasitengona) are similarly immobile during their early life, but later turn into active, predaceous animals.

Wax scale insects (?Ceroplates sp.) visited by a wasp collecting honeydew (Mozambique) [Canon 7D, Canon 180mm macro, Canon MT-24EX twin light]

A remarkably similar to ticks, but completely unrelated lineage of land parasites, are flies of the genus Forcipomyia. Some species in this group are free-living, and will fly away after taking a sip of an animal’s blood. But one, F. tettigonaris, has taken parasitism to the next level. Females of these flies find a host, usually a walking stick or a katydid, and attach themselves firmly to its body. Soon the fly turns into a permanent attachment: she loses her wings and her abdomen swells to the point where she can no longer walk. Little is known about the biology of this species, but presumably at some points she dislodges herself, falls to the ground, lays her eggs, and dies.

Ants (Crematogaster sp.) collecting honeydew from gall-like scale insects (Kermesidae)(Guinea) [Canon 1D MkII, Canon MP-E 65mm macro, Canon MT-24EX twin light]

Yet these parasites simply pale in comparison with the most advanced and most strongly modified “land polyps”, the scale insects (Coccoidea). These insects, distant relatives of aphids and other homopterans, are parasites of plants, and feed on phloem, the plant equivalent of blood (some also feed directly on the liquid content of parenchymatic cells of plants.) Their body is so incredibly modified for sedentary lifestyle that ancient Greeks and other early natural historians considered them parts of the plant (the word “Kokkos” means “a berry”.) Females of most scale insects are capable of very limited movement only early on in their development, after which time they permanently attach themselves to a plant. For the next few years, until their death, scale insects cannot move. In all species female legs are non-functional and in some the legs disappear entirely. Over time their body loses any resemblance to an insect. The female produces a large, waxy structure that hides whatever remains of her insect-like persona. In some, the structure is in a form of a flat scale, in others a round “berry”, still in others a powdery pile of wax. The only part that has contact with the outside world is her anus. That’s some serious dedication to a sedentary lifestyle.

But why is it always the females who turn into the ultimate couch potatoes? The reason has to do, of course, with the issue of reproduction. Underwater, sperm can be spread around in a cloud, which eventually will find its recipients. Some plants on land still do it. But land animals have never evolved airborne reproductive cells, and males still need to look for females, find the right opening, and do the deed themselves. In scale insects the male is nothing more than a short-lived, non-feeding, sperm-delivery system. Some scale insects do away with the males altogether, and reproduce parthenogenetically, thus completing the transition into living rocks.

An unidentified scale insect on tree bark in Mozambique [Canon 7D, Canon 180mm macro]

10 thoughts on “The ultimate couch potatoes

  1. Excellent post as always. When I first heard about the tick flies (I am pretty sure they were your photographs back then too) I thought – these are the aphids of all those leaf and stick mimics! Almighty evolution.

  2. There are a moderate number of other mite families with members that attach permanently to hosts. I believe that I recall Pyemotidae and Podapolipidae.

    Among insects there must be many more, but we can start with the Chigoe flea.

    I’m really glad you wrote about Forcipomyia: I found them when I was collecting walking sticks in Ecuador 25 years ago, but was unable to find the name for them.

  3. Mike, yes, there are many examples of sessile stages among mites, including some Uropodina or even Demodex (although these can slowly crawl if they have to.)
    The Chigoe flea female is definitely highly modified, but I also thought that they continued to move (burrow in) until the very end, but maybe not. They are pretty close to a sessile life anyway. There also must be some parasitoids, in which the larva just lies on the host and cannot move (how do larvae behave in evaniids?)

    • All of your examples were adults that actually externally attach to feed, so I was limiting myself similarly.

      There are tons of groups of mites that attach externally, but don’t feed (such as the Uropodids and the hypopal/deutonymphal stages of innumerable Astigmatids.) I don’t think that Demodex attaches: I’ve always thought of them moving around in follicles to feed off different cells.

      I suspect you’re right about chigoes.

      There are tons of parasitoid Hymenoptera that attach externally as larvae: in the Dryinidae, for example.

      Evaniid larvae do not attach to one thing: they devour the several eggs in the cockroach egg case.

  4. More sick satire presumedly.
    I was deceived for 20 years ‘mike’ – I am not a parasite. you cheated for 20 years. I was wronged.You used all of my ideas incl. multidisciplinary, grant funding, mentoring, global govts.ETC and look where you all are today. All science is unlocked.
    If it had not been for me and people from my timeline this extraordinary revolution would not have taken place. I beg to be included and not left in the rubbish tip of Australasia.
    You will be pleased to know I am tortured 24/24 via laser-x-ray optical finely focused & painful microbeams (diamond?)that have decayed my flesh and eroded my bone mass. please help.

  5. Wonderful perspective — good story telling. I have been considering a collaborative project with the hopeful title “male ants: more than just sperm missiles”. But I had never properly considered the effect of the liquid/gas medium on the adaptive landscape on fertilization. Thanks for the thought-provoking post!

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