100 NATURAL HISTORY OF WASPS. 
secretions into the cesophagus. In their structure 
the salivary glands differ from most other glands of 
the wasp, and indeed of all other insects. They are 
not tubular, but composed of separate gland-cells, 
arranged on the arborescent plan of many glands of 
the higher animals. These gland-cells are discs of 
various sizes, about a ninth of a line in diameter, on 
an average, in the hornet. They are clear and 
transparent when the insect has been kept long 
without food and in confinement; and then they 
display one or more nuclei. But, in a freshly caught 
healthy queen wasp I have sometimes seen them 
opaque and finely granular. They are each inclosed 
in a membranous bag, which contracts into a duct 
or stalk on which they hang, and which serves to 
convey their secretion to the common duct. The 
whole gland on either side might be compared to a 
bunch of grapes, or, better perhaps, of raisins; only 
the ducts are rather longer in proportion to the 
size of the gland-cells than the grape-stalks. The 
larger ducts are distinguished by their pale reddish- 
yellow colour, by the fine, soft, spiral fibre which sur- 
rounds them, and by the great thickness of their outer 
coat which is very obvious in the larger ducts. Even 
when they contain air, which they often do in small 
quantities, their colour and the thickness of their 
walls will distmguish them at once from the air-tubes 
proper with their dark sharp outlines. The smallest 
ducts have no spiral fibre, and they need careful 
adjustment of the light to display them satisfactorily, 
or indeed to shew them at all. 
The salivary glands are connected in name and 
position only with the digestive system. They have 
