4 MEDICAL ENTOMOLOGY 



External parasitism. The most important and most abun- 

 dant external parasites of man and of the domesticated animals 

 are found among the insects and arachnids. Very serious and 

 often fatal results are due to this form of irritation, and the loss 

 of blood due to an abundance of any blood-sucking species must 

 not be overlooked. External parasites may be either permanent 

 or temporary with relation to their host. The commonest 

 permanent parasites are the biting and sucking lice, which are 

 usually transferred from host to host by close association of 

 mammals while sleeping together, or in close quarters or in 

 copulation; in poultry generally while roosting. While per- 

 manent ectoparasites are not so largely concerned in the direct 

 transmission of infectious diseases, certain sucking lice are 

 known to be agents in the transmission of Trypanosomes. The 

 temporary (intermittent) ectoparasites are the most important 

 of all disease carriers, on account of their change of host, drop- 

 ping off or flying away from one animal to another of the same 

 or a different species. It may well be seen that herein lies the 

 danger of transmitting infectious diseases from animal to 

 animal. The temporary ectoparasites are well represented by 

 the fleas, bedbugs and ticks. 



Insect venoms. Another form of irritation is produced by 

 the inoculation of a specific poison into a wound produced by a 

 piercing or stinging insect. Many insects produce severe irrita- 

 tions by their bites, which fact can be accounted for by the 

 presence of a venom-secreting gland, often salivary. The cone- 

 noses or kissing bugs (Reduviidae) inflict a very painful wound 

 which is aggravated by a poison; other insects produce nettling 

 when handled, e. g. the blister beetles (Meloidae). Again, the 

 familiar sting of the bee and the wasp is chiefly painful because 

 of the injection of a specific poison. 



