Histamine (HA) receptor 1 antagonist (125 μM chlorpheniramine) and histidine decarboxylase (HDC) inhibitor alpha-methylhistidine (AMH, 100 μM) treatment of competent larvae leads to increased caspase activity. Caspase activity was analyzed using FAM-VAD-FMK, a fluorescently tagged caspase inhibitor. Normalized fluorescence was measured in the arm tips of competent larvae. HA, HA receptor 2 antagonist (200 μM cimetidine), HA receptor 3 antagonist (125 μM Thioperamide) and KCl had no effect on caspase activity. The upper panel shows representative fluorescent images of treatment categories: B-control, C-HA (1 μM), D- HA receptor 1 antagonist (125 μM chlorpheniramine), E-HA receptor 2 antagonist (200 μM cimetidine), F-HA receptor 3 antagonist (125 μM Thioperamide), G-alpha-methylhistidine (AMH, 100 μM) and H-KCl. The lower panel shows the corresponding results of the fluorescent analysis. Panel A illustrates the approximate region of the arms that was included in the analysis. Note that all fluorescent intensities were normalized to the area measured and the exposure time. Scale bars: 20 μm.