THE CHIGOE 351 



basement. In the U. S. the ground squirrel, Citellus beechyi acts as a reservoir of 

 plague and has as its flea Hoplopsyllus anomalus. 



Ceratophyllus fasciatus is the common rat flea of Europe and the U. S. In the 

 tropics X. cheopis is the common rat flea (98% in India). Ctenocephalus serrati- 

 ceps, Ctenopsylla musculi and Pulex irritans have also been frequently found on 

 both Mus norvegicus and M. rattus. To distinguish M . norvegicus from M. rattus 

 we have in the former (i) ears which barely reach the eyes when laid forward and (2) 

 tail rather shorter than length of head and body together (only 89% of length of head 

 and body together). With M. rattus the tail is longer than the head and body 

 together (25% longer) and the extended ear covers or reaches beyond the middle of 

 the eye. M. rattus has a sharper nose, longer and more delicate tail and thinner ears 

 than M. norvegicus (formerly M. decumanus). 



M. alexandrinus is a variety of M. rattus. Rats and mice belong to the family 

 Muridae and the common mouse is M. musculus. They belong to the order of 

 Rodentia of the class Mammalia. 



Sarcopsyllinae 



Belonging to the subfamily Sarcopsyllinae, the Sarcopsylla penetrans (Derma- 

 tophilus penetrans} is of great importance in tropical countries. It is known as 

 the chigoe, nigua, or jigger. The male and virgin female are unimportant as they 

 do not penetrate the skin but act as ordinary fleas. The female, which when un- 

 impregnated is only about ^4 inch long, when impregnated bores its way into 

 the skin of man, especially about the toes, soles of the feet or finger-nails, and in the 

 chosen site develops enormously, becoming as large as a small pea. This enlarge- 

 ment takes place in the second and third abdominal segments and is packed with eggs 

 measuring about 400 microns long and numbering about 100. A small black spot in 

 the center of a tense rather pale area is characteristic. The metamorphosis is 

 similar to that of the flea. Sarcopsylla can be differentiated from the flea by the pro- 

 portionately larger head to the body, and especially by the fact that the head is the 

 shape of the head of a fish, distinctly pointed. With the fleas the lower border of the 

 head comes out in a straight line to join the curve of the upper part. In the Sarco- 

 psylla lower and upper border of head are both curved. 



DlPTERA 



The insects of the order Diptera are of great importance medically 

 in a variety of ways, either by the direct irritation of their bites, by their 

 transmitting disease directly, as does the common house fly typhoid 

 fever, or by acting as intermediate or definitive hosts for various 

 parasites. They are characterized by mouth parts formed for punctur- 

 ing, sucking, or licking. They present a complete metamorphosis, larva, 

 pupa, and imago. As a rule, the Diptera have a distinct pair of wings 

 the second pair being rudimentary. 



The order Diptera is usually divided into the following suborders: i. Orthorrha- 

 pha: Diptera with larvae having a differentiated head. The imago breaks through 



