WOODLICE 11 



known as 'Weber's glands' played an important part in respira- 

 tion. For nearly half a century the concept that these glands 

 secreted a fluid which moistened the gills crept into almost every 

 text-book dealing with the subject of respiration in woodlice. 

 Great, therefore, was the surprise of Gorvett (1950), who has made 

 a special study of the glands of woodlice, to find that 'Weber's 

 glands' do not in fact exist either in the animals or in the publica- 

 tions of their supposed discoverer. Gorvett traced the myth to an 

 'inaugural dissertation' on respiration in woodlice published in 

 1909, in which glands described by Weber were confused with 

 certain structures in the abdomen that had been described, also 

 incorrectly, by Nemec in 1895-6. Apparently the accounts of 

 Weber and Nemec had never been compared, or it would at once 

 have been realised that the two authors were dealing with entirely 

 different and totally unrelated structures. 



At least five kinds of tegumental glands do, however, occur in 

 woodlice, of which the rosette and lobed glands have so far been 

 investigated by Gorvett. Some of the latter discharge an acid 

 secretion smelling of butyric acid, but others possess an odourless, 

 neutral solution. The glands vary in number and size in different 

 species, but their variation appears to be independent of habitat or 

 evolutionary position: in fact their function is probably to act as a 

 deterrent to enemies, principally hunting-spiders, since they do 

 not seem to prevent excess evaporation as has sometimes been 

 suggested. They are thus analogous to the repugnatorial glands of 

 millipedes, harvest-spiders and many insects (Gorvett, 1956). 



Woodlice do not seem to be attacked to any great extent by 

 parasitoidal insects. Thompson (1934) made a detailed study of 

 their Tachinid parasites during which he dissected 1,737 speci- 

 mens of Porcellio scaher and Onisciis asellus collected from various 

 localities in England and France but only 9-1% were parasitised, 

 and the average parasitism of O. asellus was only 3-1%. He con- 

 cluded, therefore, that Dipterous parasites are not factors of major 

 importance in the control of woodlice: certain species are scarcely 

 parasitised at all and in the case of others, starvation and cannibal- 

 ism come into play as density-dependent factorsf controlling the 



t See discussion of density-dependent and density-independent factors 

 on p. 141. 



