- Scale insect
Temporal range: Lower Cretaceous–recent 
Female cottony cushion scale (Icerya purchasi, Monophlebidae) with young crawlers Scientific classification Kingdom: Animalia Phylum: Arthropoda Class: Insecta Order: Hemiptera Suborder: Sternorrhyncha Superfamily: Coccoidea
Handlirsch, 1903 
Most scale insects are parasites of plants, feeding on sap drawn directly from the plant's vascular system. A few species feed on fungal mats and fungi, e.g., some species in the genus Newsteadia in the family Ortheziidae. Scale insects vary dramatically in their appearance from very small organisms (1–2 mm) that occur under wax covers (some look like oyster shells), to shiny pearl-like objects (about 5 mm), to creatures covered with mealy wax. Adult female scales are almost always immobile (aside from mealybugs) and permanently attached to the plant they have parasitized. They secrete a waxy coating for defense; this coating causes them to resemble reptilian scales or fish scales, hence the name.
Scale insects feed on a wide variety of plants, and many scale species are considered pests. Some types are economically valuable, such as the cochineal, Polish cochineal and lac scales. Scale insects' waxy covering makes them quite resistant to pesticides, which are only effective against the first-instar nymph crawler stage. However, scales are often controlled with horticultural oils, which suffocate them, or through biological control.
Some scale insects species evolved symbiotically with some ant species.
Scale insects show very great sexual dimorphism. Female scale insects, unusually for Hemiptera, retain the immature external morphology at sexual maturity (neoteny). Adult males can have wings but never feed and die within a day or two. Male scale insects may or may not have wings, depending on their species. Species in which males do have wings are unusual among insects, in that they generally possess only one pair of fully functional wings. In this they resemble true flies (Diptera). However, their tail filaments do not resemble anything in the morphology of flies. Their hind (metathoracic) wings are reduced, commonly to the point that they generally are overlooked. In some species the hind wings have hamuli, hooklets, that couple the hind wings to the main wings, which usually is a condition associated with the Hymenoptera. The vestigial wings often are reduced to the point where they are referred to as halteres or pseudohalteres. It is not at present clear to what extent the pseudohalteres have any substantial control function to match the true halteres of the flies.
The specifics of their reproductive systems vary considerably within the group, including hermaphroditism and at least seven forms of parthenogenesis.
The main families of scale insects are:
- Margarodidae – cottony cushion scales, giant coccids and ground pearls
- Diaspididae – armored scales
- Dactylopiidae – cochineal
- Kerriidae – lac scales
- Coccidae – soft scales
- Asterolecaniidae – pit scales
- Pseudococcidae – mealybugs
- Eriococcidae – felted scales
A number of other families are known only from fossils, including Arnoldidae, Electrococcidae, Grimaldiellidae, Grohnidae, Hammanococcidae, Inkaidae, Jersicoccidae, Kukaspididae, Labiococcidae, Lebanococcidae, Lithuanicoccidae, Pennygullaniidae, Serafinidae and Weitschatidae.
- ScaleNet homepage
- Cottony cushion scale: the pest that launched a pest control revolution
- Diaspididae of the World
- Scale Insect Forum
- Scales of southerastern U.S. woody ornamentals
- Ceroplastes rubens, red wax scale
- Ceroplastes rusci, fig wax scale
- Coccus viridia, green scale
- Eucalymnatus tessellatus, tessellated scale
- Phoenicoccus marlatti, red date scale
- ^ Johnson, M.S.; et al (2001). "Acropyga and Azteca Ants (Hymenoptera: Formicidae) with Scale Insects (Sternorrhyncha: Coccoidea): 20 Million Years of Intimate Symbiosis". American Museum Novitates 3335: 1–18. doi:10.1206/0003-0082(2001)335<0001:AAAAHF>2.0.CO;2.
- ^ "Coccoidea Handlirsch, 1903". Integrated Taxonomic Information System. http://www.itis.gov/servlet/SingleRpt/SingleRpt?search_topic=TSN&search_value=109195.
- ^ Shouhei Ueda; Swee-Peck Quek; Takao Itioka; Keita Inamori; Yumiko Sato; Kaori Murase; Takao Itino4 (2008 October 22). "An ancient tripartite symbiosis of plants, ants and scale insects". Proc Biol Sci (The Royal Society) 275 (1649): 2319–2326. doi:10.1098/rspb.2008.0573.
- ^ Scholtz, C. H. (1985). Insects of southern Africa. London: Butterworths. ISBN 0-409-10487-6.
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