Mosquitoes suck the bloods of not only humans, but animals, too.
Cows [1], and turkeys are only two example species of animals.
Some mosquitoes prefer avian blood, and target humans mainly when birds are not found around -- and their assaulting both species, is messing by carrying illnesses (such as West Nile Virus) between humans and birds. Similarly, some carry illness between monkeys and humans.
For example, in the case of Dengue fever transmission, "Infected humans are the main carriers and multipliers of the virus, serving as a source of the virus for uninfected mosquitoes. The virus circulates in the blood of infected humans for two to seven days, at approximately the same time that they have a fever; Aedes mosquitoes may acquire the virus when they feed on an individual during this period. Some studies have shown that monkeys in some parts of the world play a similar role in transmission." [4]
Likewise, in the case of yellow fever transmission, "mosquito is the primary vector. It carries the virus from one host to another, primarily between monkeys, from monkeys to humans, and from person to person." [5]
"The emergence of West Nile (WN) virus in 1999 in four U.S. states was followed by its spread to 12 states in 2000. An enzootic cycle was established between birds and mosquitoes, and WN disease was observed in humans, horses, and birds in both years." [6]
That generalization of their target-selection, is a good news, for finding out what would suffice to pull mosquitoes to a trap. If they were to look out specifically for the shape of a human, or bird, then you would have to place full-blown sculptures (or, straw figures), but it turns out that they have specific sensors,[2] through which they estimate (the suckability of) their potential victims.
[1] Brain, M. (July 21, 2005). "HowStuffWorks "DIY Mosquito Trap"." Retrieved from http://home.howstuffworks.com/mosquito-magnet1.htm, on April 16, 2009.
[2] Freudenrich, C. (July 5, 2001). "HowStuffWorks "How Mosquitoes Work"." Retrieved from http://www.howstuffworks.com/mosquito.htm, on April 14, 2009.
[3] "World Poultry - Diseases: Flies (and mosquitoes)" Retrieved from http://www.worldpoultry.net/diseases/flies-28and-mosquitoes29-d67.html, on June 16, 2010.
[4] WHO (March 2009). "WHO | Dengue and dengue haemorrhagic fever" Retrieved from http://www.who.int/mediacentre/factsheets/fs117/en/, on March 22, 2010.
[5] WHO (December 2009). "WHO | Yellow fever" Retrieved from http://www.who.int/mediacentre/factsheets/fs100/en/index.html, on March 22, 2010.
[6] Bernard,K.A., Maffei,J.G., Jones,S.A., Kauffman,E.B., Ebel,G.D., Dupuis;A.P.,II, Ngo,K.A., Nicholas,D.C., Young,D.M., Pei-Yong Shi, Kulasekera,V.L., Eidson,M., White,D.J., Stone,W.B., NY State West Nile Virus Surveillance Team, & Kramer,L.D. (Jul–Aug 2001). "CDC - West Nile Virus Infection in Birds and Mosquitoes, New York State, 2000" Emerging Infectious Diseases, Vol. 7, No. 4. Retrieved from http://www.cdc.gov/ncidod/eid/vol7no4/bernard.htm, on June 20, 2010.
listing mosquito-characteristics (mnemonic "sercan")
MosquitoCharacteristics1: Unisex Target
MosquitoCharacteristics2: Sucks Not Only Human Blood, But Animals, Too
MosquitoCharacteristics3: Surging
Why mosquitoes? Reflecting about the purposes of Allah, in creating those.