You're right of course, that absolute negatives are hard to prove. However,
even under controlled laboratory conditions, HIV failed to persist in
mosquitoes. Again, though, the most compelling argument against arthropod
transmission is the epidemiology. When you think about it, no one has ever
conclusively proven that HIV-1 causes AIDS, simply because Koch's postulates
cannot be satisfied with this virus in humans. However, the epidemiological
evidence is overwhelming, and all but a select few now believe that HIV does
indeed cause AIDS.