Mechanism of locomotion observed on caterpillar hairs.

To the Editorial Committee of the BRITISH JOURNAL OF OPHTHALMOLOGY. SIRs-Although ophthalmia nodosa has been known for more than a century (Sch6n, 1861), the mechanism of the migration of caterpillar hairs has remained controversial. There is no need to enumerate the explanations proposed, but one point should be noted here which has rarely been mentioned in the recent literature, not even in the excellent paper by Watson and Sevel (1966). Pathologically, it has long been known that caterpillar hairs entering the conjunctiva or cornea become surrounded by a granulation nodule consisting of giant cells, epithelioids, and lymphocytes; these infiltrates develop at the broken-offend of the hair from which a toxic substance leaks into the surrounding tissue. The undamaged tip of the hair is not surrounded by these granulations unless, for some reason, the hair remains in the tissue for a longer period of time. While the soft conjunctival and episcleral tissues permit the formation of a protruding nodule, the stiffer cornea and sclera do not expand; this mechanical difference together with the shape of the broken-off hair led to the conclusion (Ascher, 1937) that the cellular infiltrate pressing against the broken-offend of the hair pushes it forward in the direction of least resistance, i.e. with the sharp unbroken tip forwards and in the same interlamellar space unless the sharp tip happens to enter a neighbouring interlamellar space; then, the hair will continue to be pushed forwards parallel to the fibres of this lamella. Some authors have assumed that barbs might prevent the hair from wandering with the undamaged tip forwards, but a convincing experiment was performed by Geserick (1954): under the corneal microscope he used a keratome to divide a caterpillar hair imbedded in the cornea; the proximal end of the hair remained immobile, surrounded by an infiltrate of oval shape while the distal sharp end continued to migrate forwards, being apparently pushed in that direction by a triangular exudate at its cut-off end. This experiment has confirmed the explanation given 30 years ago and has refuted the assumption that the barbs would interfere with the movement of the hair in the direction of its natural tip. Yours faithfully, KARL W. ASCHER. 1404 EAST MCMILLAN STREET, CINcINNATI, OHIo, 45206, U.S.A. November 4, 1967. REFERENCES ASCHER, K. (1937). Med. Klin., 33, 297. GESERICK, H. (1954). Zbl. Chir., 79, 764. -