INSECTS AND HUMAN DISEASE 143 



adhere to one another, so that, when magnified, they resemble 

 miniature bunches of grapes. They are ovoid in shape, and 

 each egg has a smooth, soft, and highly refractile covering. 

 The average time for incubation is twenty days. At the 

 end of seven days the egg changes its shape, becoming 

 more ovoid and transparent, so that the larva can be seen 

 within the shell. On or about the thirteenth day the 

 shell splits and discloses a six-legged larva which is 

 incapable of locomotion, despite the fact that it can move 

 its legs freely. Then a very peculiar thing happens : instead 

 of the larva emerging from its covering, as is the usual 

 case, it remains within its shell, beneath which air enters, 

 so that the young tick appears of a dull white colour. A 

 close examination at this period will show that the dull 

 purplish-brown larva is about to moult, and, within its 

 skin, the eight-legged nymph may be seen. Eventually 

 the yellowish-coloured nymph crawls backwards from the 

 combined egg shell and larval skin. Nymphs do not feed 

 for three or four days, but when once they have started 

 to feed they grow quickly, if they are provided with a 

 plentiful supply of food. After three moults the adult stage 

 is reached, when, despite the absence of eyes, no time is 

 lost in seeking human blood. 



It has been shown by experiment that an unfed female 

 weighing '0270 grm. will weigh '2602 grm. after a meal, 

 an increase of approximately ten times her original weight. 

 Remarkable as this may seem, it is by no means a record, 

 for the gorged females of some species of ticks increase to 

 thirty times their original unengorged weight. Imagine a 

 hungry 10-stone man weighing 300 stones after a single 

 meal, or even after many meals ! Ornithodorus moubata 

 transmits relapsing fever by reason of its feeding habits ; 

 the spirochsetes are not found in the tick's salivary glands, 

 but are voided in the excrement at the time of feeding, 

 and so pass into the wound, and infection takes place. The 

 process is not merely mechanical, for a developmental 



