Very Small Insects
Embioptera
The Web Spinners
Web spinners, or embiids, are a primitive group of insects. Although the earliest fossil specimen dates from the Permian period, they probably evolved earlier, in the Carboniferous period. They are about 5-12mm (0.2-0. 5") and live gregariously, hidden in silken tunnels networked to form a colony. Perhaps related to the Orthoptera, the Embioptera have a number of adaptations not found in any other insects which hinder the establishment of phylogenetic relationships. The mouthparts are directed forward rather than downward, quite unlike other primitive Orthopterans. This may be an adaptation to life in a tunnel or may mean that Embioptera are actually related to earwigs (Demaptera). Most striking of all are the tarsi of the front legs. These are enlarged and contain glands that produce silk. No other group of insects, fossil or modern, have silk-producing glands in the legs. This silk is used to construct elaborate nests and tunnels under leaves or bark. Web spinners live within these silken nests, feeding on all manner of vegetable matter. They rarely leave their network of tunnels, the colony growing through incorporation of new food sources. The ordinal name Embioptera is derived from the Greek words “embio” (lively) and “ptera” (wing). The name references the fluttery movement of wings observed in male Embioptera. Only adult males have these wings, of which there are two pairs. Front and hind wings are similar in shape and unusually flexible and can fold forward over the head when the insect runs backward through its tunnels. Hemolymph is pumped into anterior veins to stiffen the wings during flight. Male web spinners only live for a short time and those species with wings fly around in search of females (who sometimes eat their suitors). After mating, female web spinners either lay their eggs in an established gallery or disperse to begin a new colony. The female protects her eggs and, in some species, feeds the nymphs upon hatching. The nymphs resemble the adults and live together with their extended family, spinning silken galleries of their own. Females of different species are so similar in appearance that it is often impossible to tell them apart. Male web spinners may sometimes be attracted to lights at night. Most Embioptera are tropical or subtropical. There are 200+ species.
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Phthiraptera
The Parasitic Lice
The Phthiraptera were once treated as two separate orders (Anolplura and Mallophaga) but are now recognized as one. The ordinal name Phthiraptera is derived from the Greek words "phthir" (lice) and "aptera" (wingless). Calling them “wingless lice” is appropriate, for no member of this order requires these appendages. Living on the skin of their many hosts, ranging from birds to humans, parasitic lice live very specialized existences. There are many, many different kinds of lice, the most well known (of course) being those that live on humans. The three species that live on humans have been able to divide the human body up amongst themselves; one species living in the hair on the head, one crawling about on the body and one living in the pubic region. Human body lice are actually closely related to human head lice; the two species diverged when humans began to wear clothing, therefore creating a new habitat niche! Lice can be divided into two groups: those that have mandibles and chew (Mallophaga) and those that have mouthparts designed to suck body fluids (Anolplura), the former probably giving rise to the latter. Anolplura is now considered a suborder within Phthiraptera. The old taxon Mallophaga is paraphyletic and now recognized as three suborders: Ischnocera, Amblycera and Rhyncophtherina. Unlike many other ectoparasites, the Phthiraptera cannot survive long if separated from the body of their host. Eggs (nits) are glued directly to hair or feathers and nymphs hatch and feed on the parental host. Since lice have no wings, dispersal to new host animals is limited solely to occasions when members of the host species come into direct contact. A close association with its food supply means that most lice are limited to a very narrow host range - often only a single species. Through speciation, there are now about 3,000 species of parasitic lice. Parasitic lice appear to be a recent phenomenon, geologically speaking. Until quite recently, the oldest specimens known were removed from the hair of 10,000-year-old wooly mammoths frozen in Siberian permafrost. An exceptionally preserved, 44 million year-old fossil louse was recently discovered in Germany. Remains of the louse’s last meal, feathers, can be seen preserved within its gut. The fossil louse is remarkably similar to feather-eating lice found on some modern-day aquatic birds. This fossil shows that birds have been infested with parasitic lice since at least the Eocene epoch, raising the possibility that lice were passed on to birds from early, feathered dinosaurs.
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Psocoptera
The Booklice and Barklice
The order Psocoptera is comprised of the booklice and barklice. They are not true lice, but resemble them superficially. The ordinal name Psocoptera is derived from the Greek words, “psoco” (rub small); and “ptera” (wing). This references the tented appearance of the wings when held at rest, on those species that have them that is. These "lice" are tiny scavengers, thriving in microhabitats at the size level of mold. They are so small (1-10mm /0.04-0.4") that they feed on pollen, spores, single-celled algae, and lichens, as well as all manner of plant and insect tissue fragments. They evolved from the early Hemiptera, with the earliest known fossil Psocoptera occurring in the Permian period (225-280 million years ago). Psocopterans, perhaps, later gave rise to the true parasitic lice. The booklice are the smallest of the Psocopterans. Capable of eking out a living almost anywhere, they can be found inside and around human dwellings where they feed on the detritus of humanity. Their diet varies, from pollen to skin cells, with some species fond of paper and flour products, giving rise to the common name "booklice". They seldom occur in large numbers but outbreaks can occur in poorly maintained or undisturbed stored products (like those encyclopedias rotting in your basement/garage/attic). The barklice are the outdoor equivalent of booklice. Although larger than booklice, and often winged, barklice are far less noticeable because they live on the bark and foliage of trees and shrubs, as well as beneath stones and ground litter. Some live communally, but due to their tiny size they generally go unnoticed, as they cause no harm. There are about 2,000 known species of Psocopterans.
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Siphonaptera
The Fleas
Somewhere in the dim past, perhaps about 160 million years ago, fleas evolved from their Mecopteran ancestors. Comprising the order called Siphonaptera, the ordinal name means “wingless siphon”. This description is due to their habit of being bloodsuckers who have lost the power of true flight and live on or near their host, a habit that has made fleas a “big” nuisance. Heard of the Black Plague that killed off a third of Europe’s population in the 14th century? That was caused by a bacterium (Yersinia pestis) carried by fleas (who transmitted the disease to humans), which were distributed via rats. Fleas are quite small, 1-9mm (0.04-0. 33"), with bodies flattened on the vertical. They have backward-pointing bristles on their bodies and "swim" through the hairs on their hosts. Even more impressive is their ability to easily jump a hundred times there own body length at any given moment. This is allowed because of a fabulous internal structure called a pleural arch, constructed of an even more amazing material, called resilin. Resilin is an elastic protein that is extremely efficient, releasing as much as 97% of stored energy! This energy is directed into the legs, which, in preparation for “flight” are held raised up against the body. Upon unlocking of the trochanters, the flea will accelerate 200 gravities, faster than the human eye can perceive! This apparatus is quite mechanical, requiring no potential muscle energy and capable of performing just as well in the freezing cold as in the blazing heat. A flea can jump 600 times or more in an hour while it searches out a host! Like mosquitoes, they can track the carbon dioxide signatures of their hosts. There are about 2,000 known species, only ten percent preying on birds (mostly seabirds and a few perching birds, especially swallows and martins, with 19 species of fleas associated with those two birds alone!). The rest feed on mammals. Very few of humankind’s furry brethren have escaped the piercing stylets of fleas. A few species that seem to often be flea-free are: aquatic mammals, flying lemurs, zebras, rhinos, aardvarks and primates. Fleas generally rely on mammalian hosts who build nests, for they have a larval stage dependent on their parents for blood nourishment.
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Strepsiptera
The Twisted-wing Parasites AKA Stylopids
Truly the most obscure and bizarre of all the insects, twisted-wing parasites have not yet been classified to the agreement of all scientists. Placed in the order called Strepsiptera, the word translates literally to “twisted wing” and references the strange twisted shape of the wings while a male is in flight. It is only the males that bear these wings. They have two pairs, consisting of forewings greatly reduced to look and function like the halteres of flies, and sparsely veined, fan-shaped hind wings. Completing it’s bizarre appearance, males have flabellate, branched antennae, and raspberry-like eyes; the latter very unusual among living insects and forming a modern counterpart to the structural plan proposed for the eyes of trilobites! Most of the known species have only been identified from specimens of the males. Females are much harder to find and are probably often overlooked since they live as grub-like parasites inside their host insect. Devoid of eyes, antennae, legs and external genitalia (except in the family Mengenillidae), the fused head and thorax projects from the host. With a greatly reduced mouth and a single genital opening, sex pheromones are emitted to attract the free-living males. After mating, fertilized eggs are retained within the female who later gives birth to six-legged larvae. The mobile larvae will go out and seek new hosts, dropping off the host insect that bears their mother as it travels about. Once another host insect is found, the larvae burrow inside, feeding until they reach maturity. They then pupate within the still living host. Later, males will emerge to seek out the females (which means that the host may now become permanent residence for several females). Though twisted-wing parasites do not feed on their host’s vital organs, they do greatly reduce nutrient intake and can affect the size of body parts, often causing the genitalia of the host to be so reduced as to render the individual sterile. So far, it is known that they parasitize silverfish (Lepismatidae), cockroaches (Blattidae), mantids (Mantidae), crickets (Gryllidae, Gryllotalpidae, Tettigoniidae), pygmy mole-crickets (Tridactylidae), plantlice (Psyllidae), Auchenorrhynchans (Cercopidae, Cicadellidae, Membracidae, Delphacidae, Dictyopharidae, Eurybrachidae, Ricaniidae, Flatidae, Fulgoridae, Issidae, Tettigometridae), true bugs (Coreidae, Cydnidae, Lygaeidae, Pentatomidae, Scutelleridae), flies (Tephritidae, Platysomatidae) and a wide range of Hymenopterans (Formicidae, Mutillidae(?), Vespidae (Masarinae, Eumeninae, Polistinae, Vespinae), Sphecidae, Colletidae, Halictidae, Andrenidae). Once thought to be beetles, they also share traits with flies. Strepsipterans do not appear in the fossil record until the Cretaceous (136-64 million years ago). There are about 500 species known, with the greatest number occurring in the tropics.
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Thysanoptera
The Thrips
Thrips are very tiny insects, measuring only 0.5-1.5mm (0.06-0.2"). Thrips are so small that they feed on individual fungal spores or use their unique, asymmetrical mouthparts to pierce and feed on individual plant cells. They have chewing mouthparts but the right mandible has been lost and the left one is used as a piercing tool (thrips are the only insects that have such asymmetrical mouthparts). Most species feed on plant sap but some live in leaf litter and feed on fungi; some are actually predatory, feeding on other insects. The ordinal name Thysanoptera comes from the Greek words “thysanos” (fringe) and “'pteron” (wing). This “fringe” is created by setae, a design unique to the Thysanoptera that greatly increases the surface area of their wings. Interestingly, they are often reluctant fliers with a predisposition towards flying before thunderstorms; hence they are called “thunder-flies” in some places. They are easily blown about on the wind and use it for dispersal. Winged and wingless forms may occur within the same species, the wingless forms tending to be more common in autumn. Most species of thrips over-winter as either adults or pupa. They are described as holometabolous insects (exhibiting complete metamorphosis) even though the nymphs look like small, wingless adults. Thrips undergo an extended metamorphosis in which the final, immature stage (consisting of 2 or 3 inactive pupa-like instars) is quiescent, non-feeding, and sometimes even enclosed in a silken cocoon. This stage of development, usually called a pupa, has aroused a great deal of speculation by some entomologists who claim that thrips represent a missing link between hemimetabolous and holometabolous development. A close examination of Thysanopteran pupae reveals that they do not undergo any internal transformation. It may well be that these "pupae" are an example of convergent evolution instead of a “missing link”. Debates aside, it is known that thrips often have short life cycles; from egg to adult, it may take just three weeks. Because of this, they can easily reach plague proportions and may occur in such large numbers that they become pests in commercial agriculture. Damage from extensive feeding can cause stunted and deformed fruits and seeds. While some thrips are pests and known carriers of plant diseases, others are important pollinators of the flowers they feed on. One species of thrips (Liothrips urichi) has been successfully used to control a pest plant (Clidemia hirta) in Fiji. Most fossil thrips have been found in amber, where they are fairly common. The oldest fossil thrips are from the Permian period, about 280-225 million years ago. The word "thrips" is both singular and plural (there is no such thing as a "thrip"). There are about 5,000 described species.
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Zoraptera
The Zorapterans AKA Angel Insects
An obscure and tiny group of insects (only about 30 known species), the order Zoraptera was described in 1913. These insects are quite small, measuring no more than 3-4 mm (0.1”). Found in damp, rotting wood, sawdust and leaf litter, they are most abundant in the tropical and subtropical areas of the world. Once thought to be absent from Australia and Europe, these strange insects are turning up in many new places, including the nests of some termites and mammals. The ordinal name Zoraptera is derived from the Greek words "zor", meaning pure, and "aptera", meaning wingless. The name was given before winged forms were discovered. These winged individuals are rather uncommon and may be dispersal forms. Zorapteran wings break off easily, near the base, leaving only stubs. It is thought that Zorapterans evolved from primitive orthopterans and share an ancestry with ancient cockroaches. Others see them as relatives of the termites (Isoptera) since they can voluntarily shed their wings. Some entomologists believe they may represent an evolutionary link between the orthopteroids and the hemipteroids (particularly Psocoptera). Much research needs to be done, for little is known about them. They live in small aggregations but do not appear to have any social structure. They have biting mouthparts and scavenge on spores, fungal mycelium and/or mites and other small arthropods. Fossil Zoraptera are very hard to come by. Four species, representing both alate and apterous forms, were recently found preserved in Cretaceous amber from Myanmar (Burma). These are the first fossil records of the order from the Old World and also the oldest known so far, dating to about 100-90 million years ago. Prior to this discovery, fossil Zoraptera specimens were known only from two species dating to the Miocene, found in amber from the Dominican Republic.
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