62 Proceedings 
their habit during the daytime. When full-fed the caterpillars 
are from 7 to 8 centimeters long and about a centimeter thick, 
and a cluster will form a ball as big as ones fist. This brightly 
coloured ball swinging about among the foliage must be easily 
seen by birds and other enemies. But there is a very strong and 
disagreeable ‘choky’ sort of smell about the caterpillars that 
probably prevents the approach of such. 
In the genus Papilio there is adistinct organ on the prothoracic 
segment of the caterpillar to produce the offensive odour, a V- 
shaped fleshy fork that can be protruded at will and is highly 
charged with a pungent liquid of a very unpleasant smell; in 
most of the Brazilian species that I have observed it is something 
like a mixture of rotten oranges and acetic acid. In the case of 
Papilio thoas the caterpillars rest during the daytime in large 
clusters on the trunks of the orange, their food-plant; and when 
disturbed and the scent organs are exerted the smell could be 
‘cut with a knife.’ 
In the Notodont species, Aneurocampa mingens possesses the 
protection of offensive odour in perfection, being capable of 
directing a fine spray of pungent liquid from the mouth exactly 
on to the spot where it is touched by an offending object. This 
must be an unfailing weapon of defence against insect enemies 
such as ichneumons as well as against birds or mammalians. The 
liquid smells exactly like acetic acid and has a strong acid re- 
action, as tested by blue litmus. 
I have observed the same habit in a closely allied species, 
Psorocampa dentilinea, Schaus. 
The caterpillars of many families, e.g., Avctiadae, Lyman- 
triadae, Lasiocampidae, etc., are clothed with barbed hairs 
which are easily detached or broken when handled, and cause 
considerable irritation when brought into contact with the skin; 
a very effective protection against birds and other devouring 
enemies, though they would not interfere with ichneumons. 
But the most highly specialised weapon of defence that I have 
observed occurs in the Saturniadae, the Ceratocampidae, the 
Megalopygidae and the Limacodidae, whose caterpillars are 
furnished with hollow needle-pointed spines that inject a viru- 
lent poison“into the skin of an attacking enemy. The poison is 
