none The larvae of Croesus varus are cryptic and those of C. septentrionalis aposematic. In both species, females lay eggs in groups, but the groups formed by young larvae are smaller in C. varus than in C. septentrionalis . In both species, group size decreases during successive larval instars, and the cryptic species becomes solitary, while the aposematic species remains gregarious during all larval instars. On Alnus shrubs in the field, the distribution of the groups of C. septentrionalis larvae is more aggregative. They are found mainly on medium-sized shrubs (1.5-2.5 m high), whereas groups of C. varus larvae are also found on larger shrubs. Moreover, during the 2 years of observation, C. septentrionalis was present for a shorter time than C. varus , even when the former was much more abundant than the latter. Thus, conspicuousness of the aposematic species should be amplified by this spatial and temporal aggregation. Colour and luminescence; Colour; Markings; Integument; Nutrition; Diet; Behaviour; Agonistic behaviour; Social behaviour; Parasites diseases and disorders; Parasites; Insect parasites; Hosts; Insect hosts; Ecology; Land and freshwater zones; Palaearctic region; Europe Croesus septentrionalis (Tenthredinidae); Warning coloration; Integumentary glands; Ventral gland size & defensive efficiency of secretions; Defensive behaviour; Larval strategies & efficiency; Aggregating behaviour; Gregariousness; Hymenopteran parasites; Excavarus apiarius & Rhinotorus atratus; Host defence; Plant hosts; Alnus glutinosa & A. incana (alder); Larval defence strategy, distribution & gregariousness; Predators; Formicidae; Prey defence, strategies & efficiency; Distribution within habitat; Larvae, defensive strategy efficiency significance; Belgium; Rochefort; Larval defence strategies, gregariousness & distribution, plant host Croesus varus (Tenthredinidae); Cryptic coloration; Integumentary glands; Ventral gland size & defensive efficiency of secretions; Defensive behaviour; Larval strategies & efficiency; Aggregating behaviour; Gregariousness; Hymenopteran parasites; Excavarus apiarius & Rhinotorus atratus; Host defence; Plant hosts; Alnus glutinosa & A. incana (alder); Larval defence strategy, distribution & gregariousness; Predators; Formicidae; Prey defence, strategies & efficiency; Distribution within habitat; Larvae, defensive strategy efficiency significance; Belgium; Rochefort; Larval defence strategies, gregariousness & distribution, plant host Excavarus apiarius; Rhinotorus atratus (Ichneumonidae ); Hymenopteran hosts; Croesus septentrionalis & C. varus; Host defence, strategies & efficiency, plant host; Belgium Formica fusca; Lasius brunnells; Myrmica rubra (Formicidae ); Prey; Croesus septentrionalis & C. varus; Prey defence, strategies & efficiency, plant host; Belgium