δ13C‰ and δ15N‰ values of spermathecae from the control experiments, treatment 1. The four symbols indicate samples from the 4 replicates. Threshold values were defined as 2–3 standard deviations (s.d.) above the mean value of virgin spermathecae samples, and are indicated for δ15N‰ (2 s.d. solid line) and δ13C‰ (3 s.d. broken line). Experimental samples could thus be defined as 13C labelled (top left quadrant), 15N labelled (bottom right quadrant), labelled with both isotopes (Both, top right quadrant) or no label was detected (Neither, bottom left quadrant).