NO. 7 



PROTECTIVE ADAPTATIONS — INIcATEE 



17 



Since arthropods are by far the most important phyktm of animals 

 as bird food and since it is with reference to the inckided class of 

 insects that the theories of protective adaptations have been most 

 highly elaborated, it is desirable to subdivide the phylum for the pur- 

 poses of the present discussion. 



The tabulation below shows the number of identifications and their 

 percentages for the four classes of Arthropoda that are available for 

 food to nearctic birds. 



Class 



Crustacea 



Myriapoda . . . . 



Insecta 



Arachnida 



It is evident tliat tlie percentage of identifications for each class 

 I corresponds very well with the frequency of such animals as indicated 

 by the number of species. The validity of such comparisons used 

 throughout this paper thus receives further corroboration. 



Crustacea (Crabs, Shrimps, Sowbugs) 



Protective adaptations. — The exoskeleton of Crustacea is either 

 chitinous or much thickened or calcified. Several groups have bivalved 

 carapaces in which the animal is nearly or entirely enclosed. The 

 barnacles have a more complicated covering of calcareous plates, some- 

 times thick and hard. ATost of the decapods have strong grasping fore- 

 legs, and have furthermore the faculty of snapping these off when 

 properly stimulated. Some Crustacea have burrowing habits (Hip- 

 pidae) and others (hermit crabs) use the shells of univalves for 

 shelter, while the terrestrial sowbugs roll themselves up into a ball 

 when disturbed. 



As to color, some of the smaller a(|uatic forms are translucent or 

 transparent ; the ostracods are said to assimilate with the general 

 color of their environment, while some copepods and decapods are 

 brilliantly colored. As to form, Mortensen says: " 1 he typical cases 

 of adaptation to life among algae are especially found among the 

 Caprellids ; they might be said to represent the Phasmids and Geo- 

 metrid larvae, among marine animals " (p. yy). " Idothca marina and 



