scent is rather like that given off by Cockroaches."[1] "When a Dytiscus is captured, it often discharges a milky fluid from the thorax, just behind the head. The fluid smells like sulphuretted hydrogen.[2] The Y-shaped "horn," which can be projected from near the head of the larva of Papilio machaon, is the source of a powerful odour of fennel—one of the food-plants of the caterpillar.[3] The scents emitted by insects cannot be always estimated as of a "protective" character. Barrows and Schwarz have stated that "it would seem that Crows have a predilection for insects possessing a pungent or otherwise strong taste or colour." This is exemplified by the prevalence of Carabidæ (among them the often-recurring genus Chlænius possessing a peculiar odour), coprophilous or necrophagous Coleoptera (Silphidæ, Histeridæ, and Scarabæidæ, Laparosticti), ants, and more especially by the almost constant occurrence of certain species of the heteropterous family Pentatomidæ. "It seems probable that the strong odour or taste of these soldier bugs is the reason why they are so eagerly sought by the Crows."[4]
Gilbert White recorded an instance which appears to have a "protective" explanation. "I knew a gentleman who kept a tame snake, which was in its person as sweet as any animal while in good humour and unalarmed; but as soon as a stranger, or a dog, or cat came in, it fell to hissing, and filled the room with such nauseous effluvia as rendered it hardly supportable."[5] The same author remarks that "odours also appear to serve among animals as individual recognition signs. After ewes and lambs are shorn, there is great confusion and bleating, neither the dams nor the young being able to distinguish one another as before. This embarrassment seems not so much to arise from the loss of the fleece, which may occasion an alteration in their appearance, as from the defect of that notus odor, discriminating each individual personally; which also is confounded by the strong scent of the pitch and tar wherewith they are newly
