Page "Endosymbiont" Paragraph 15

Scientists classify insect endosymbionts in two broad categories, ' Primary ' and ' Secondary '.

Primary endosymbionts ( sometimes referred to as P-endosymbionts ) have been associated with their insect hosts for many millions of years ( from 10 to several hundred million years in some cases ), they form obligate associations ( see below ), and display cospeciation with their insect hosts.

Secondary endosymbionts exhibit a more recently developed association, are sometimes horizontally transferred between hosts, live in the hemolymph of the insects ( not specialized bacteriocytes, see below ), and are not obligate.