While Toll signalling in flies serves a dual purpose in development and immunity, the signalling process activated by peptidoglycan recognition protein (PGRP)-LC and Imd is specific for antimicrobial defence and is dispensable for normal development (Hultmark, 2003) . Via the NF-κ B-like transcription factor Relish, this signalling induces transcription of all major antimicrobial effector peptides in Drosophila . In Drosophila , Imd signalling is often said to be specific for Gram-negative bacteria, although Gram-positive bacteria with diaminopimelic acid-type peptidoglycans are at least as strong as elicitors. A weaker response is also seen to other types of peptidoglycan and even to fungi (Hultmark, 2003; Werner et al ., 2003; Stenbak et al ., 2004) . This broad specificity is caused by the three alternative splice forms of Drosophila PGRP-LC, which carry different peptidoglycan recognition domains (Werner et al ., 2003; Mellroth et al ., 2005 . Chang et al ., 2006 . Interestingly, the Imd signalling pathway is highly conserved in the honey bee, with plausible orthologues for all components (Fig. 2) . While this strongly suggests that Imd signalling is similar in flies and bees, it does not necessary imply similar biological roles.