Vasodilators
Vasoconstriction is very effective for closing off small vessels. Degranulating platelets release, among other things, thromboxane A2, which is a locally controlled vasoconstrictor (Vander et al, 1998). Locally controlled vasoconstrictors are released at the injury site and are self regulating, not externally regulated by nerves or off-site produced hormones. Because vasoconstriction is an immediate response to injury all biting arthropods possess vasodilators of some sort; those with shorter feeding times possessing the most potent. Vasodilators not only prevent constriction, they allow increased blood flow (hyperemia) and vessel permiability and motility . The latter results in small vessels moving closer to the skin surface (erythema: redness) and "leaking" into the interstitial spaces. These effects contribute to ease of vessel access, and the speed of formation and increased size of haematomas.
In arthropods, vasodilators are not conserved across classes, that is, there is a lot of variability, indicating late evolutionary development. One of the most powerful vasodilators described belongs to a primitive group of telophagic feeding flies known as the phlebotamine sand flies. These flies are the carriers of Leishmaniasis, a protozoan disease second only to malarial in importance to human health. The substance, whose activity was first described by Ribeiro et al (1989), is a peptide now named maxadilan. It acts similarly to human CGRP (calcitonin gene related peptide), but has been shown to be at the very many times as potent, making it, in fact, the most powerful vasodilator medically known. The authors also showed that in the presence of maxadilan, interferon was unable to stimulate complete macrophage activity in monocytes. This allowed the parasite to be phagocytosed by these cells but not killed; the protozoan then proceeds to replicate in the macrophage (!). Vasodilators in other arthropods are somewhat less interesting; ticks use a human-like prostoglandin E2, kissing bugs release a Nitric Oxide stimulating compound, which is a neurally produced vasodilator. Members of the mosquito family Culicidae uses at least two different enzymes (Beaty and Marquardt, 1996) including sialokinins. This is a very unconserved group of compounds.
