Abstract: Scanning electron microscopy was used to investigate the palpi and tarsi of Coccinella septempunctata to identify the principal chemosensory organs likely to be responsible for pesticide residue detection. The study confirmed that the range of sensilla on the maxillary palps included two types of basiconic sensilla, which are both mechanosensory and chemosensory, and one type of campaniform sensilla. The paper reports the first accurate morphometrics of these sensilla, highlighting sexual dimorphism. Measurements of the terminal segment of the maxillary palp showed a significant interaction between side (left or right lateral) and sex of the insect. A new campaniform sensillum was described for the labial palps, totalling between 12 and 17 located on each labial palp. Biometric measurements established a mean diameter of 2.4 μm for these mechanoreceptors. The only chemoreceptors on the labial palps were confirmed to be the basiconic sensilla described in earlier studies. The numbers of these basiconic sensilla were shown for the first time to be sexually dimorphic, with a mean of 18 in males and 16 in females. Sex differences were also exhibited in the tarsi: in the width of the adhesive pad of the second tarsomere, which was larger in males towards the front of the insect and in females towards the rear of the insect. Explanations for these disparities, and for the function of the newly identified sensilla, are discussed.

Key words: Coccinella septempunctata, scanning electron microscopy, ultrastructure, adhesive setae, morphometrics