Zoraptera

The insect order Zoraptera, commonly known as angel insects, contains small and soft bodied insects with two forms: winged with wings sheddable as in termites, dark and with eyes (compound) and ocelli (simple); or wingless, pale and without eyes or ocelli. They have a characteristic nine-segmented beaded (moniliform) antenna. They have mouthparts adapted for chewing and are mostly found under bark, in dry wood or in leaf litter.

Description
The name Zoraptera, given by Filippo Silvestri in 1913, is misnamed and potentially misleading: "zor" is Greek for pure and "aptera" means wingless. "Pure wingless" clearly does not fit the winged alate forms, which were discovered several years after the wingless forms had been described.

The members of this order are small insects, 3 mm or less in length, that resemble termites in appearance and in their gregarious behavior. They are short and swollen in appearance. They belong to the hemimetabolous insects. They possess mandibulated biting mouthparts, short cerci (usually 1 segment only), and short antennae with 9 segments. The abdomen is segmented in 11 sections. The maxillary palps have five segments, labial palps three, in both the most distal segment is enlarged. They have six Malpighian tubules, and their abdominal ganglia have fused into two separate ganglionic complexes. Immature nymphs resemble small adults. Each species shows polymorphism. Most individuals are the apterous form or "morph", with no wings, no eyes, and no or little pigmentation. A few females and even fewer males are in the alate form with relatively large membranous wings that can be shed at a basal fracture line. Alates also have compound eyes and ocelli, and more pigmentation. This polymorphism can be observed already as two forms of nymphs. Wingspan can be up to 7 mm, and the wings can be shed spontaneously. When observed, wings are paddle shaped and have simple venation. Under good conditions the blind and wingless form predominates, but if their surroundings become too tough, they produce offspring which develop into winged adults with eyes. These winged offspring are then able to disperse and establish new colonies in areas with more resources. Once established, future generations are once again born blind and wingless.

Phylogeny
The phylogenetic relationship of the order remains controversial and elusive. At present the best supported position based on morphological traits recognizes the Zoraptera as polyneopterous insects related to the webspinners of the order Embioptera. However, molecular analysis of 18s ribosomal DNA supports a close relationship with the superorder Dictyoptera.

The following cladogram, based on the molecular phylogeny of Wipfler et al. 2019, places Zoraptera as the sister group of Dermaptera (earwigs); Zoraptera and Dermaptera together form the sister group of the remaining Polyneoptera:

Classification
The Zoraptera are currently divided into two families, four subfamilies, nine genera and a total of 51 species, some of which have not been yet described. There are eleven extinct species known as of 2017, many of the fossil species are known from Burmese amber.
 * Family Zorotypidae
 * Subfamily Zorotypinae
 * Zorotypus — 7 spp.
 * Usazoros — 1 sp.
 * Subfamily Spermozorinae
 * Spermozoros — 6 spp.
 * Family Spiralizoridae
 * Subfamily Latinozorinae
 * Latinozoros — 3 spp.
 * Subfamily Spiralizorinae
 * Spiralizoros — 12 spp.
 * Centrozoros (=Meridozoros ; Floridazoros ) — 8 spp.
 * Cordezoros — 1 sp.
 * Scapulizoros — 1 sp.
 * Brazilozoros — 3 spp.

Incertae sedis
The following nine species are considered Zoraptera incertae sedis:
 * Zorotypus congensis – Congo (Dem.Rep.)
 * Zorotypus javanicus – Indonesia (Java)
 * Zorotypus juninensis (considered a synonym of Centrozoros hamiltoni ) – Peru
 * Zorotypus lawrencei New, 1995 – Christmas Island
 * Zorotypus leleupi – Ecuador (Galapagos Islands)
 * Zorotypus longicercatus – Jamaica
 * Zorotypus newi (=Formosozoros newi, is in actuality an immature earwig ) – Taiwan
 * Zorotypus sechellensis – Seychelles
 * Zorotypus swezeyi – United States (Hawaii)

Extinct taxa

 * Zorotypus Silvestri, 1913
 * Subgenus Zorotypus Silvestri, 1913
 * Zorotypus (Zorotypus) absonus Engel, 2008 – Dominican amber, Dominican Republic (Miocene)
 * Zorotypus (Zorotypus) denticulatus Yin, Cai, & Huang, 2018 – Burmese amber, Myanmar (Cretaceous)
 * Zorotypus (Zorotypus) dilaticeps Yin, Cai, Huang, & Engel, 2018 – Kachin amber, Myanmar (Cretaceous)
 * Zorotypus (Zorotypus) goeleti Engel & Grimaldi, 2002 – Dominican amber, Dominican Republic (Miocene)
 * Zorotypus (Zorotypus) mnemosyne Engel, 2008 – Dominican amber, Dominican Republic (Miocene)
 * Subgenus Octozoros Engel, 2003
 * Zorotypus (Octozoros) acanthothorax Engel & Grimaldi, 2002 – Kachin amber, Myanmar (Cretaceous)
 * Zorotypus (Octozoros) nascimbenei Engel & Grimaldi, 2002 – Kachin amber, Myanmar (Cretaceous)
 * Zorotypus (Octozoros) cenomanianus Yin, Cai, & Huang, 2018 – Kachin amber, Myanmar (Cretaceous)
 * Zorotypus (Octozoros) hudai Kaddumi, 2005 – Jordanian amber, Jordan (Cretaceous)
 * Zorotypus cretatus Engel & Grimaldi, 2002 – Kachin amber, Myanmar (Cretaceous)
 * Zorotypus oligophleps Liu, Zhang, Cai & Li, 2018
 * Zorotypus robustus Liu, Zhang, Cai & Li, 2018
 * Xenozorotypus Engel & Grimaldi, 2002
 * Xenozorotypus burmiticus Engel & Grimaldi, 2002 – Kachin amber, Myanmar (Cretaceous)

Behavior and ecology
Zorapterans live in small colonies beneath rotting wood, lacking in mouthparts able to tunnel into wood, but feeding on fungal spores and detritus. These insects can also hunt smaller arthropods like mites and collembolans. Much of their time is spent grooming themselves or others.

Centrozoros gurneyi lives in colonies which range in size from a few dozen to several hundred individuals, but most often number about 30 individuals. The males are slightly larger than the females, and they fight for dominance.

When two colonies of Usazoros hubbardi are brought together experimentally, there is no difference in behavior towards members of the new colony. Therefore, colonies in the wild might merge easily. Winged forms are rare. The males in most colonies establish a linear dominance hierarchy in which age or duration of colony membership is the prime factor determining dominance. Males appearing later in colonies are at the bottom of the hierarchy, regardless of their body size. By continually attacking other males, the dominant male monopolizes a harem of females. The members of this harem stay clumped together. There is a high correlation between rank and reproductive success of the males.

Latinozoros barberi lack such a dominance structure but display complex courtship behavior including nuptial feeding. The males possess a cephalic gland that opens in the middle of their head. During courtship they secrete a fluid from this gland and offer it to the female. Acceptance of this droplet by the female acts as behavioral releaser and immediately leads to copulation.

In Spermozoros impolitus, copulation does not occur, but fertilization is accomplished instead by transfer of a spermatophore from the male to the female. This 0.1 mm spermatophore contains a single giant sperm cell, which unravels to about the same length as the female herself, 3 mm. It is thought that this large sperm cell prevents fertilization by other males, by physically blocking the female's genital tract.

Effects on ecosystem
Zorapterans are thought to provide some important services to ecosystems. By consuming detritus, such as dead arthropods, they assist in decomposition and nutrient cycling.

General references

 * Costa JT 2006 Psocopera and Zoraptera. In: Costa JT The other Insect Societies. The Belknap Press of Harvard University Press, Cambridge, MA and London, UK pp 193–211