Re: what chemical does a mosquito inject before drinking blood, why does it nee

It looks like the second part of your question got chopped off, so I'll 
just answer the first part. I looked in a variety of texts, and most were 
very unhelpful, merely stating that mosquitos inject saliva. "The Science 
of Entomology", by Romoser and Stoffolano, 1998, went a bit further.

The major problems mosquitos face are blood coagulation, platelet 
aggregation, and vessel constriction. They have produced a variety of 
salivary secretions to overcome these host responses. The 
book mentioned only a few, including prostaglandins and nitrovasodilators 
to keep blood vessels open, and an enzyme called salivary apyrase to 
prevent platelet aggregation. 

Interestingly, some disease organisms interfere with this system for their 
own benefit. The critter that rides inside mosquitos and carries bird 
malaria reduces the mosquito's production of apyrase. This causes the 
mosquito to bite a bird longer, giving the parasite a better chance of 
moving from the mosquito to the bird!