Wednesday, October 19, 2011

King Harvest Has Surely Come

Leiobunum rotundum, feeding. Note the missing second leg on the animals left side. This is an example of autopasy, a defensive strategy involving losing an appendage along a preformed brake plane in response to stress such as a predator.

Everybody knows them, but few know much about them: Daddy Long Legs. Because of this lack of knowledge, much of what is known is superstition and folklore. Even the very name we give them, 'Daddy Long Legs', suggests a monstrous, lurking menace rather than the fascinating and ancient creatures that they are.
Daddy-Long-Legs (or Harvestmen) are arachnids of the order Opiliones and with over 5000 species described they are the third largest group of arachnids after the Acari (mites and ticks) and Aranae (spiders) (1). Their body has two basic regions; a prosoma (or cephalothorax) which carries all the appendages and eyes, and a limbless opisthosoma (or abdomen) which has the spiracels and genital opening. Although they resemble spiders, the two body regions are not clearly defined and they thus lack the constriction or waist that can be seen in spiders. Opiliones also lack spinning organs and so do not spin webs. They are in fact much more closely related to scorpions and pseudoscorpions (2) and are among the oldest of arachnids, with fossil records showing little morphological change in the group in 400 million years (1). They possess many characteristics that are unique to their group. These include the presence of paired trachae, the presence of a penis or spermatopositor organ in males and an ovipoistor in females and scent glands that are present on the prosoma (3). These glands release chemicals during defensive behaviour that can repel other arthropods (4). Other than these chemicals, opiliones do not produce venom and the commonly held belief that they are poisonous is untrue.
They are found on every continent, bar Antarctica, in moist to wet habitats and are excellent indicators of undisturbed environments where they feed on arthropods, snails and worms. These they ingest in particles, unlike many arachnids which need to liquify their food, and also may eat vegetable matter (2). Most species in the Northern Hemisphere overwinter as eggs, hatching in the spring. Maternal care is common in opiliones, as it is in many other arachnid groups, but uniquely so is paternal care. Adults reach maturity in the Autumn, which has lent them one of their common names, Harvestmen. Many species are very tolerant of other individuals, again unlike other arachnid groups, and often large aggregations of adults and subadults can be seen near water sources.
One of the most conspicuous of the 18 species of opiliones present in Ireland (15 native, 3 introduced and naturalised) (5) is the longlegged Leiobunum rotundum. Preferring open spaces, L. rotundum feeds on a variety of invertebrate prey as well as soft fruits (6). The individual pictured is mising one of its legs and thus shows an example of autopasy (also called autotomy). This is a situation where an outside agent is responsible for the severance of an appendage at a preformed breakage plane (e.g., the loss of limb to a predator who has seized it) (7). The limb will not grow back.

  1. Pinto-da-Rocha et al., 2007. Harvestmen: the biology of Opiliones
  2. Giribet et al., 2002. Cladistics 18 pp. 5-70
  3. Giribet, 2009. Encyclopedia of Insects pp. 247-248
  4. Hara et al., 2005. Biochemical Systematics and Ecology 33 1210-1225
  5. Ferriss et al., Irish Biodiversity: a taxonomic inventory of fauna pp. 83-84
  6. Höfer et al., 2002. Arthropod Structure and Development 29 pp. 13-21
  7. Maginnis, 2006. Behavioral Ecology 17 pp. 857-872

No comments:

Post a Comment