HARVEST-SPIDERS 141 



changes of temperature and humidity: but according to one school 

 of thought there must be some biological density-dependent factor 

 to keep their numbers in check. For if a particular component of 

 the environment is to regulate the population density of a species, 

 it must be able to destroy a greater fraction of the population of that 

 species when the density is high than when it is low. If destruction 

 were merely proportional to population density, the factor causing 

 it would not be a regulatory one. Parasites and predators are ex- 

 amples of the first category (density-dependent factors), cli- 

 matic conditions of the second. Other writers suggest that natural 

 control is achieved chiefly by climatic and edaphic factors and that 

 it is unnecessary to invoke density-dependent factors to explain 

 either the maximum or the minimum number occurring in a 

 natural population. Perhaps the truth may lie somewhere between 

 these extremes. t 



The following predators have been recorded: fish (when harvest- 

 men have been caught by the sudden flooding of streams), frogs, 

 toads, lizards, birds, shrews, badgers, foxes and other mammals, 

 centipedes, spiders, predaceous flies, beetles, earwigs, dragonflies, 

 bugs and other insects, as well as cannibalistic Opiliones (Sankey, 

 1949a). However, the ground-living forms are exceedingly incon- 

 spicuous and probably seldom found by their potential enemies, 

 whilst the long-legged species generally have cryptic coloration, 

 and can often stride out of harm's way. If caught they may escape 

 by autotomising a limb. The leg breaks at the articulation between 

 the coxa and trochanter and there is no bleeding. The detached 

 portion may make spontaneous rhythmical movements which per- 

 sist for an hour or more and serve to distract the predator's atten- 

 tion from its prey. Lost appendages are not regenerated however, 

 and harvest-spiders appear to lack the ability to rid themselves of 

 damaged members but continue to drag a crippled leg until it 

 severs itself. It is interesting to note here that autotomy does not 

 appear to occur in the Trogulidae. 



Should a harvestman come to grips with an aggressor a fluid is 

 extruded from its repugnatorial glands that is distasteful to most 

 invertebrate predators. Thus few spiders will sustain an attack on 



t I have recently discussed these points; 1957, Entomologist, 90, 

 195-203. See also; Milne, A. (1957) Canad. Ent. 89, 193-213. 



