138 HARVEST-SPIDERS 



Like other Arachnids, harvestmen carefully clean their limbs. A 

 leg is held in the chelicerae which open and shut as the long seg- 

 ments are pulled through them. By the time the tarsus is reached, 

 the leg is bent almost into a circle, and finally shoots out like an 

 unbent spring. The pedipalps are cleaned in the same way and the 

 chelicerae are washed while drinking. A few species are able to 

 stridulate but the sound is probably too faint to be audible to the 

 human ear. 



To the lyriform organs — button-shaped slits found singly or 

 together in a lyre-shaped group — have been attributed the function 

 of smell. They occur especially on the first segments of the cheli- 

 cerae and on the coxae of the legs. Each is supplied with a nerve 

 and they may well be proprioceptors analagous to the chordotonal 

 sensillae of insects which serve to detect strains and stresses in the 

 integument. Harvestmen respond to touch and chemical stimuli, 

 and bunch together in captivity. Their eyes are simple and prob- 

 ably serve mainly for distinguishing light from darkness. It is 

 doubtful if there is much form-vision in any of the Opiliones but 

 some perception of movement may occur. Although harvest- 

 spiders are susceptible to changes of temperature and humidity, 

 the organs which serve these functions are not yet known. Todd 

 (1949) has worked out the temperature and humidity preferences 

 for some British species and found a strong correlation between the 

 humidity preference of the species tested in the laboratory and 

 their stratification in oak woodland. At the same time it was noted 

 that those species with the highest temperature preference ap- 

 peared to have a southern distribution or to live in drier or warmer 

 habitats than others. 



British harvest-spiders can be divided into various groups 

 according to their vertical distribution. Thus the species which 

 usually live on the ground under stones, logs and in moss or plant 

 debris include those with short legs and small eyes such as Trogulus 

 tricartnatus, Anelasmocephalus cambridgei, Nemastoma lugubre, 

 N. chrysomelas, Homalenotus quadridentatus, Oligolophus meadii and 

 Opilio saxatilis. The second group includes species which live 

 mainly amongst low vegetation (the field layer) of grass and other 

 herbaceous plants, but whose young stages occur on the ground, 

 such as Nelima silvatica^ Leiobunum blackwalli^ L. rotundum, Mito- 



