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.
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.
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.
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.
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.