The order Zoraptera is one of the least diverse and most poorly understood of insect lineages. Zoraptera are minute, hemimetabolous insects superficially resembling barklice (Psocoptera) or termites (Isoptera). Individuals are generally less than 3 mm in total body length, exclusive of their antennae, and are gregarious. Overall, zorapterans are quite homogenous in their morphology and tend to be off-white (mostly nymphs) to brown in color. Individuals of each species occur in two morphs: eyed and winged forms that then shed their wings after dispersal or blind and apterous forms that predominate in colonies. The order consists of a single family, Zorotypidae, presently with two recognized genera. Only one, very preliminary attempt has been made to elucidate phylogenetic relationships among zorapteran species (Engel, 2003b). A complete monograph of the order has been in preparation for many years and should be finished in the next 3-4 years (Engel, unpubl.)
Zorapterans live in small colonies of 15-120 individuals which they found in the crevices or under dead bark of moist, decaying logs (they will also colonize man-made sawdust piles). Species feed principally on fungal hyphae and spores but can also be general scavengers or predators, victimizing nematodes, mites, or other tiny arthropods. Other aspects of their biology and behavior are considered by Choe (1994a, 1994b, 1995, 1997), Engel (2003a), and Valentine (1986).
Although considered rare by many entomologists, once a successful "search-image" is developed for locating zorapteran habitats, then species are typically not difficult to locate. In fact, the "rarity" of zorapterans may be merely apparent and more a result of poor collecting than any actual scarcity. Certainly some species are uncommon, but overall the actual abundance of the order likely does not live up to its reputation for scant occurrence (this is particularly true for the North American species, Zorotypus hubbardi: Engel, pers. obs.). Wilson (1959) went so far as to identify a "Zoraptera stage" in decomposition whereby logs can be easily torn with an ordinary collecting tool. Naturally-formed spaces in logs not reached by light harbor zorapteran colonies. Once the wood is disturbed individuals will quickly scatter to avoid detection, thus fogging with a general insecticide prior to dissecting a log can aid collection. Individuals are best preserved in ethanol.
Zorapterans are principally distributed pantropically with only four species occurring North of the Tropic of Cancer (i.e., North of 23.5°N): Z. hubbardi in the United States, Z. snyderi in Florida, and Z. sinensis and Z. medoensis in Tibet. Almost all of the northernmost records for Zorotypus in North America are from sawdust piles rather than natural logs suggesting that the distribution in regions with considerable winter frosts is artificially influenced by human activity (Engel, 2003a, 2004). The effects of global climate change on northern migrations of Zorotypus are uncertain but are likely to permit species to expand their distributions significantly in North America and Asia. Aside from the aforementioned four species, all zorapterans are tropical. Even those fossil records of Zoraptera are all from ambers formed in warm-tropical paleoclimates (Engel and Grimaldi, 2002). While species have at times been considered highly endemic and hypothesized to have poor dispersal abilities, increasingly individual species are being discovered to have larger geographic ranges than previously understood (e.g., New, 2000; Engel, 2001). Such species provide evidence of some degree of dispersal ability in zorapterans so as to maintain specific integrity over these ranges. Furthermore, the presence of zorapterans on distantly isolated islands of relatively recent geological age such as Hawaii indicates some dispersal capabilities.
Angel Flies have one family (Zorotypidae) and one genus (Zorotypus). They are polymorphic, but have similar off-white to brown coloration. They are about three millimeters long. They have short antennae, short cerci, and two-segmented tarsi. They feed primarily on fungus spores. At times they can be scavengers or hunters, feeding on small arthropods, nematodes, or mites. Some species have membranous wings and most do not. The winged species will shed their wings as adults. They can be found in colonies of 15-120 in rotten logs and underneath bark. They prefer dark habitats and are nearly blind. When their habitat is disturbed, they quickly scatter and hide. They are abundant in their specific habitats and are rarely found elsewhere. Three species can be found in the United States. They are seen in the fossil record as far back as the early Cretaceous. They preferred tropical weather in the early Cretaceous and all species but four live in tropical climates now.
The order is clearly ancient, perhaps diverging from the Embiodea during the Late Triassic or Early Jurassic. Relatively modern-looking species of both orders are known from middle Cretaceous amber (Engel and Grimaldi, 2002; Grimaldi et al., 2002). Indeed, Cretaceous zorapterans already exhibit the development of dual morphs (eyed alates versus blind, apterous individuals) within the species. Thus, typical zorapterans (even belonging to the moder genus Zorotypus) were already established by the middle Cretaceous (about 100 million years ago). Unfortunately, earlier records of Zoraptera will be challenging to find since these minute, delicate insects require remarkable fidelity in preservation to allow meaningful identification, something best achieved in amber. Presently, however, insect-bearing ambers are known only back as far as the earliest Cretaceous and it may be decades before Late Triassic or Jurassic zorapterans are recognized.
Phylogenetic Position of Zoraptera
Although there are numerous hypotheses concerning the phylogenetic placement of Zoraptera (e.g., Hennig, 1981; Boudreaux, 1979; Rasnitsyn, 1998; Wheeler et al., 2001), a conclusive placement of the order remains controversial and elusive. At present the most well-supported position recognizes the Zoraptera as polyneopterous insects (admittedly with numerous reductions in structures from the typical polyneopteran groundplan: Grimaldi and Engel, 2005) and as sister to the webspinners [order Embiodea (= Embioptera, or Embiidina)] (Engel & Grimaldi, 2000; Grimaldi & Engel, 2005). This conclusion is based on numerous shared apomorphic traits the most notable being the unique hyper-development of the metatibial depressors in both orders, dehiscent wings, reduced anal region of wings (i.e., narrow, paddle-shaped wings), gregarious behavior, and apterous morphs (occurs among some male Embiodea). Additional shared traits are discussed by Engel and Grimaldi (2000).
The order can be characterized as follows (from Engel, 2003a, 2004): adults minute (around 3 mm), hemimetabolous Neoptera; mouthparts mandibulate; lacinia fused to stipes, lacinia with strong inner, apical tooth; maxillary palpus five-segmented; labial palpus three-segmented; distal palpal segment of labial and maxillary palpi larger than preceding palpal segments; prementum divided; antennae nine-segmented (except in Octozoros where it is eight-segmented), moniliform; compound eyes and ocelli present in winged forms; lateropleurite and laterosternite differentiated in alates; wings membranous with reduced venation or wings frequently absent, forewings narrow and paddle-shaped owing to reduced anal lobe, forewing with radial, median, and cubital veins fused at base; hind wings smaller than forewings; all wings dehiscent, shed by indefinite basal fracture; coxae large; metafemora stoutly expanded, with stiff spines along ventral surface, metatibial depressors greatly developed (as contrasted with the greatly developed metatibial levators of saltatorial lineages such as Orthoptera); tarsi two-segmented, first segment minute, second segment elongate; claws simple; abdomen 11-segmented; two abdominal ganglia; cerci present, short, unsegmented (except in Z. goeleti a weak, distal second segment is evident); ovipositor absent; male genitalia asymmetrical, gonostylus absent; female with 4-6 panoistic ovarioles; six Malpighian tubules; gregarious.Some of the more notable derived features of zorapterans include:
- reduced wing venation, in paddle-shaped wings that are shed at basal fractures.
- gregarious lifestyle, with individuals living in colonies up to about 120 individuals.
- polymorphism in populations; with blind, apterous individuals dominating during colony life, but populations produce eyed alates (individuals with wings) for dispersal and founding new colonies.
- two-segmented (dimerous) tarsi.
- enlarged metafemora (hind femurs), with rows of distinctive spines along ventral surfaces.
- males with a distinctive "mating hook" and unique mating behaviors.
- unique grooming behaviors (Valentine, 1986).
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