98 CRUSTACEA. 



pair of antennae. Abdomen composed of many segments. Legs very 

 numerous, always more than eight pairs. Tracheal respiration. 

 4. Inseota — Head, thorax, and abdomen distinct. One paii' of antennse. 

 Eyes both simple and compound. Legs always six in number. No 

 appendages of locomotion ever on the abdomen. Two pairs of wings 

 generally present. Respiration tracheal. 



Crustacea. 



The Crustacea are chiefly aquatic arthropods. Some few 

 species live in damp earth and in damp general surroundings : 

 these belong to the order Isopoda. The order Isopoda contains 

 the Wood-lice : they may be told by the equal and symmetrical 

 feet or legs, whilst in the others, such as the Crab and Lobster, 

 the claws are asymmetrical. There are three species injurious 

 to horticulturists generally, all of which belong to the family 

 OniscidcB. The Oniscidae belong to the 

 second tribe of Isopoda, called the 

 Euisopoda, which have the body made 

 up of seven free thoracic segments and 

 the same number of paired appen- 

 dages. The abdominal feet are so 

 modified as to form branchial lamellae. 

 The Oniscids or Wood-lice (fig. 37) are 

 land Isopods, with the endopodites 

 Fig. sT.-wooD-LioE. ^f ^.^g abdominal feet formed into 



ceiiiosraber;°c%^™igaris;°Di branchiae, the exopodites into protec- 

 ^prosiC"'""'= ^' '"'*''"" tive lateral plates on the outside. 

 Peculiar modifications may be seen on 

 the two front abdominal feet, which are provided with special 

 sacs to contain air. The three common species are Oniscus 

 asellus of Linnaeus, Porcellio scaler of LatreUle, and Armadillo 

 vulgaris of Linnaeus (fig. 37). They are popularly called 

 Monkey-peas or Slaters in Britain, the Cow-bugs of America; 

 perhaps the term Wood-louse is that most usually employed. 

 Oniscus asellus is omnivorous, eating animal and vegetable 



