The coloration of Amblyomma hebraeum, A. gemma, Dermacentor venustus, D. variabilis and D. reticulatus niveus, as seen in living examples, is depicted for the first time, and the desirability of recording the colours of ornate ticks when alive is indicated.
A remarkable change in colour in living specimens of A. hebraeum is described and figured, this change taking place after a prolonged sojourn (74–141 days) upon the host. Such a colour change has not hitherto been observed in ticks. The difference in colour is seen in dead dried specimens but is not appreciable in those preserved in alcohol.
Since the immature stages of ornate ticks are inornate, and the colours change in adults of some species during prolonged periods of parasitism, it is evident that the coloration in adults must depend upon the accumulated products of metabolism beneath the chitinous exoskeleton, the regional distribution of colour depending upon special metabolic functions taking place in corresponding parts of the tick. The ornamental colour-producing layer can be scraped away from the underside of the scutum in most ornate ticks and it is removable from such ticks by the use of caustic potash.
The whitish or creamy coloration that is so characteristic of most species of Dermacentor and the three ornate species of Rhipicephalus that are known to science, appears on the other hand to depend largely upon inclusions or structural changes within the chitin itself, whence the persistence of the creaminess seen by reflected light in the caustic-treated ticks. Similar, but less distinct, appearances may be seen in ticks belonging to other genera. Dermacentor rhinocerotis, which does not exhibit creamy coloration but only dull yellowish spots when dry, is totally decolorized by caustic potash, whilst contrary to most species of its genus it shows metallic coloration in well-preserved specimens in alcohol.
The examination of caustic-treated specimens was carried out upon 31 species of ornate ticks, i.e. Dermacentor (8), Rhipicephalus (3), Amblyomma (15), Aponomma (4) and Hyalomma (1 species).
The coloration and creamy ornamentation, herein distinguished, appear to be confined to the thinner portions of the exoskeleton. That the characteristic dark markings correspond to thicker and more darkly chitinised portions of the scutum is demonstrable by dissections, caustic-treated, or sectioned specimens; this being especially evident for instance in Amblyomma. The orange spot on the scutum of A. splendidum offers an exception.
The optical and chemical study of coloration in ticks deserves further investigation.