The terms ctenidia and combs can be used interchangably for spines on the head/thorax. Spines on the cheeks are referred to as genal and spines on the thorax are called pronotal combs. The presence and location of the chitinous spines are important in differentiating between the different flea species. The chitinous anal struts are also used for identification and help the flea larvae with vigorous motility.