The insect order of Lepidoptera ("scaly winged") derives its name from the fact that the adult animals are covered with numerous small scales. The larvae or caterpillars are covered with hairs. Broadly speaking, this order can be grouped into butterflies (antennae club-shaped) and moths (antennae pointed or with a feathery ending). The mouthparts of most butterflies consist of a thin tube (coiled tongue) with which they can suck nectar. In many butterflies, the mouthparts are atrophic. Primitive butterflies (Micropteryx sp.) still have biting mouthparts. Some species drink tears or suck blood (e.g. Calyptera sp.).

There are numerous species of caterpillars whose hairs have an irritant effect when they come into contact with the skin. Various species belonging to the Zygaenoidea, Bombycoidea ("silk moths"), Sphingoidea ("hawk or sphinx moths"), Notodontoidea, Noctuoidea ("owl moths"), Geometroidea ("spanners") and Papilionoidea ("pages, morphos") cause dermatitis. Various species of Saturniidae ("Atlas moths") are renowned for the discomfort they cause. The dermatitis due to Hylesia metabus, H. alinda and H. paulex tends to be severe. Pararama is the name for the disorder caused by Premolis semirufa, a Brazilian caterpillar belonging to the Arctiidae ("tiger moths"). Permanent skin lesions and peri-arthritis occur, for example in workers in rubber plantations. Megalopyge operculata, M. superba and M. lanata (flannel moths, belonging to the Zygaenoidea) are also well-known causes of epidemic episodes of pruritic dermatitis. The symptoms are similar to those caused by the urticating hairs of bird spiders. A haemorrhagic syndrome can develop as a result of contact with the fibrinolytic poison of the caterpillars of Lonomia achelous ("giant silkworm moth"). The Butantan Institute in Brazil has developed an antidote.

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• Brown tail moth caterpillar, Euproctis chrysorrhea, belonging to the Lymantriidae. The caterpillar’s food plants are blackthorn, hawthorn and various fruit trees. The insect was previously accidentally introduced into the New World so that this animal now causes problems in the USA.
• Drinker moth caterpillar, Philudoria potatoria, family of Lasiocampidae ["lappet moths"]. The sobriquet refers to the fact that the caterpillar sometimes drinks from drops of water on leaves. The caterpillar feeds on various large grasses.
• Oak eggar, Lasiocampa quercus (family Lasiocampidae, superfamily of Bombycoidea). The caterpillars possess simple urticating hairs which cause a transient rash. They live predominantly on heaths and other open terrain, sometimes in light woods. The caterpillars are found on heather, bramble and hawthorn.
• Oak processionary caterpillar, Thaumetopoea processionea. These animals cause considerable problems. The related species Thaumetopoea pityocampa (pine processionary caterpillar) is found in the Mediterranean Sea area. These also have urticating hairs and sometimes create a severe local plague.

As an example we will discuss the oak processionary caterpillar here in greater detail.

The oak processionary caterpillar (Thaumetopoea processionea) belongs to the superfamily of Notodontoidea, family of Thaumetopoeidae. Until 1987 this insect was rare in the low countries, but this changed markedly at the end of the 20^th century. The caterpillars come out in April and pupate around the end of June/July. From mid-July to mid-September the adults emerge. These grey-white nocturnal moths lay their eggs in small packages on the young shoots of various trees and preferentially in the crowns of common oaks. The caterpillars have the habit of living in a group and thus being present in large numbers at one site. They make typical nests against stems and thick branches. The caterpillars have a blue-grey colour dorsally and are somewhat more green-grey ventrally. On their body they have long white hairs with a red base. The nests contain numerous urticating hairs. The caterpillars can cause considerable damage to different oak species. Birches, acacias, hawthorn, mountain ash and beech are also damaged.

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From the third larval stage, the young caterpillars have numerous urticarial hairs, with microscopic barbs. The hairs are very small (100-200 µm). They are light, easily detached and can therefore be carried over large distances by the wind. The hairs form a natural means of defence against predators. They contain the protein thaumetopoin. After penetration in the body this protein stimulates the release of histamine and probably other chemical mediators. Numerous red itchy spots appear which resemble an allergic reaction. Vesicular reactions are possible. Irritation by these caterpillar hairs is known as erucism. Inhalation of the urticating hairs can have troublesome to serious consequences (nose and throat pain with dysphagia and dyspnoea). Sometimes there are systemic reactions. If the hairs enter the eyes, local inflammation occurs (keratitis or nodular conjunctivitis).

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It should always be ensured that any hairs remaining on clothing and skin are removed. For mild skin irritation, local disinfection and symptomatic therapy with antihistamines is sufficient. Sometimes topical or systemic steroids are necessary. If hairs enter the eye, mechanical removal is essential, which is followed by anti-inflammatory and antibiotic eyedrops.

Caterpillars can be controlled by selective or non-selective insecticides [e.g. Bacillus thurigiensis var. kurstaki, diflubenzuron (Dimilin®, an ecdysone analogue), teflubenzuron], by mechanical removal and by aspiration or burning. If the hairs of dead caterpillars are released, they can still cause problems. Trees can be injected with abamectin or acephate. These substances are distributed over the plant via the sap circulation and the caterpillars die after eating the leaves. Extracts obtained from the neem tree (Azadirachta indica, family of Meliaceae) have also been tested. This plant contains among other things azadirachtin which probably acts as an anti-ecdysoid, as a result of which metamorphosis is impeded. The natural enemies of caterpillars include ichneumon wasps such as Pimpla turionellae (fam. Ichneumonidae) and tachinid flies (fam. Tachinidae), e.g. Pales processionea, Carcelia iliaca. Another important natural enemy is a beetle, Calosoma sycophanta. The latter, however, is at present very rare. The use of pheromones is experimental. Pheromone traps based on the attractive substances of a related species have not succeeded. Theoretically it might be possible to design insecticides on the basis of a species-specific virus (e.g. baculovirus).