The urticating apparatus in the caterpillar of Lonomia obliqua (Lepidoptera: Saturniidae)

• Location of Lonomia obliqua venom-producing cells in the spines.
• Morphological variation of the spines in the same scolus.
• Different secretions in the spines of the same scolus.

The presence of specialized cells for venom production in the Lonomia obliqua caterpillar has long been a controversial topic. In this study, we identify a cell inside the spine that specializes in the production of toxins. Our histological study showed that this glandular cell was inserted at the subapical region of the spine, in a constricted region like a ring. This cell type was not observed in all spines of the scolus. The constricted region of the spine observed by scanning electron microscopy displayed a circular groove in which the apical portion of the spine fits perfectly; however, some spines in the same scolus lacked this groove. After breaking off the spine at the most apical region, a small drop of orange or green liquid was observed to flow from its tip. These secretions were analysed by MALDI-ToF and found to possess biochemically different compositions. The green secretion demonstrated greater similarity to the haemolymph of the caterpillar than the orange secretion. Based on our findings, the spines with a groove probably contain the venom glands and produce an orange secretion. However, it is also possible that both secretions play an important role in envenoming because all spines in contact with the skin of the accidental victim should break regardless of whether they are present in a groove.