Myrmecophilous and termitophilous interactions likely contributed to the competitive advantage and evolutionary success of eusocial insects, but how these commensal and parasitic relationships originated is unclear due to absence of fossil records. New extinct cockroaches of the still living family Blattidae are reported here from the Cretaceous Myanmar amber (99Ma) and are the earliest known inhabitants of complex ant nests, demonstrating that this specialised myrmecophily originated shortly after ant eusociality and appeared in the fossil record. Cretaceous stem aposematic Blattidae are known from the amber of Myanmar and Lebanon and we report them here also from the Syrian amber. Concurrent evolution suggests that the collective internal defence of early ants was weak and allowed infiltrations by numerous unrelated organisms, At the same time, the contemporary presence of ant mimicking myrmecomorphs suggests a need for strong external protection against visually hunting predators. Myrmecophily is supported by morphological adaptations (lack of wide fat body and feeding of adult male; short, fossorial legs; shortened cerci; oligomerised antenna; hairy surface structures) and camouflage behaviour, documented by sediment and own feces covering. Moreover the same piece of amber contains ants, ant mimics and other undescribed ant nest-visiting insects as syninclusions. Another species preserved along with two termites is a putative termitophile. Abundant comparatively large parasitic cockroaches influenced Mesozoic tropical forest ecosystems by affecting the early evolution of complex nests of eusocial insects. Rainforest rudiments in South Yunnan yielded observation of analogical still living, formally undescribed species.