
Appearance
Syromastus rhombeus is the only species with such a broad and distinctly diamond-shaped abdomen. Nymphs have very pointed abdominal tergites, each bearing a strong spine.Length 9.5-10.5 mm

Naming
SynonymsCimex quadratus Fabricius, 1775
Cimex rhombeus Linnaeus, 1767
Syromastes rhombeus (Linnaeus, 1767)
Syromastus rhombeus f. sinuata (Fieber, 1861)

Behavior
Adults emerge and mate in the spring; nymphs can be found June-August feeding on sandworts, spurreys and other plants in the Caryophyllaceae.References:
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https://www.britishbugs.org.uk/heteroptera/Coreidae/syromastus_rhombeus.html