Butterfly caterpillars

Caterpillars (butterfly caterpillars) are the larvae of butterflies or some other insects. The caterpillar is the actual feeding stage of the butterfly. The caterpillars usually lead a hidden life and are also well adapted to their environment. They usually have a green or brown colouring. As the body volume of the caterpillars increases considerably during their physical development, they shed their skin several times until they have reached their final size.

The caterpillars of butterflies consist of evenly spaced segments that form the trunk. They can be subdivided into the 3 areas head, chest and abdomen. The head is usually hardened by chitin deposits. On the underside outside they usually have six point eyes each (stemmata). In contrast to the stump-shaped feelers, the mouth parts are strongly pronounced.

Medically important is the so-called caterpillar dermatitis, which can be found among forest and garden workers, but also among walkers and residents.

The caterpillars of the butterfly go through five to six stages of development until the finished butterfly. From the 3rd larval stage onwards, the caterpillars carry fine, barbed hairs on their backs ("stinging hairs", Setae, 0.2 mm). These fall off already with light touch. They can be transported aerogenically over long distances. The older caterpillars (5th to 6th stage) in particular pose a danger, as they form up to 5 million stinging hairs. But also the cocoons of the spinner caterpillars contain stinging hairs and can cause dermatitic reactions even years later. These mainly affect the uncovered skin areas such as neck, face and unclothed extremities. Contact with stinging hairs containing a toxin (thaumetopoein) causes a toxic-irritant dermatitis.

Processional spinners can also cause allergies of the immediate type (type I reaction). In the case of the jaw processionary moth (Mediterranean distribution) 7 allergens have been described. Inhaled stinging hairs can cause irritation of the upper respiratory tract, and with the appropriate preload, also respiratory distress (10-11% of contacts).

Eyes: 15-20% of the contact persons develop conjunctivitis, often with severe swelling of the eyelids. Anaphylactoid reactions up to anaphylactic shock have also been described. The duration of the disease is usually 1-2 weeks, but symptoms may still be present a month later.

Almost every affected person (98 %) complained of itching of the skin 10-12 % complained of bronchitic symptoms.

Poisonous caterpillars, such as many species of the bear moth (Arctiidae), warn predators by their conspicuous colouring. Some species develop social behaviour. For example, the caterpillars of the processionary moth (Thaumetopoeidae) live together in large webs and move together in long "processions" to their food sources. Because of this form of camouflage, predators are not able to recognize a single caterpillar.

For years the thermophilic oak processionary moth (EPS, Thaumetopoea processionea) has been on the advance in Europe. It can be found in northern France, Belgium, Holland and England and in the warmer areas of Germany (especially in Brandenburg, Berlin, North Rhine-Westphalia, Baden-Württemberg and Franconia). The northward spread was made possible by a decrease in late frosts, which synchronized the hatching of EPS larvae.

The Oak Processionary Moth owes its name to the fact that the caterpillars are mainly found on oak trees. After hatching, they go in a kind of procession in search of food.

1. Rodriguez-Mahillo AI et al (2012) Setae from the pine processionary moth (Thaumetopoea pityocampa) contain several relevant allergens. Contact dermatitis 67:367-374.
2. Schöllnast R et al. (2004) Edematous papules and papulovesicles in mother and child. Dermatologist 55: 480-481
3. Utikal J et al (2009) Caterpillar dermatitis. An increasing dermatologic problem in warm regions of Germany. The dermatologist 60: 48-50.
4. Wang D et al (2019) Itchy rash caused by the oak processionary caterpillar. Ned Tijdschr Geneeskd 163. pii: D4243.