THE AUSTRALIAN NATURALIST. eae) 1) 
; Z 
along the sea-shore. Probably its larva is a kelp-feeder, and your 
large concourse of flies may have been due-to a loeal plethora of 
successful larvae that had been feeding on some local accumulation 
of old scaweed, possibly buried or hidden near the rocks you saw 
them on. lies of this sort are quite fond of basking on rocks, sand, 
sea-weed, ete.’? 
I can only add to what Dr. Tillyard says that there is 
exceedingly little kelp lying about the Wollongong beaches 
at the place where I saw the flies; the caretaker gathers 
up and removes it regularly. I did not ask what was done 
with the kelp, and it is quite probable that it may be buried, 
as Dr. Tillyard suggests, in which case it might form an 
ideal incubation place for the flies. Specimens of such flies 
should be preserved while alive, preferably in the field, in 
70 per cent. methylated spirit. 
TRACKS OF GARDEN SNAIL.* 
_ By Thos. Steel, BLS. 
In the Victorian Naturalist, XXXIV., p. 171, Profes- 
sor Sir Ba'dwin. Spencer asks oe information regarding 
the track left by the common introduced garden snail, Helix 
aspera, which is in the form of a series of short, thick, de- 
tached lines or patches. I have made a number of obser- 
vations on the subject, and think I have arrived at the 
correct explanation. 
The mucus which forms the track appears to be dis- 
charged intermittingly from the orifice of the large mucus 
eland near the anterior end of the ventral surface, and its 
purpose is to serve as a lubricant for the smooth passage 
of the animal’s body. As the animal moves forward the 
mucus is thus deposited in isolated portions, with the result 
- that on a rough surface, such as asphalt, most of the mucus 
remains where deposited, and is not spread forward to any 
extent. On a smooth surface, such as glass, the mucus is 
carried forward, through the animal’s ambulacral surface 
being in close contact with the glass, and so a continuous 
trail is formed. When snails crawl on rough surfaces, such 
as asphalt, wood. or a hard earth footpath, the mucus is 
frequently carried across from one deposit to another in the 
form either of a thin film or a mere thread, thus forming a — 
series of connections or bridges between successive dis- 
charges. ; 
—~ 
* Victorian Naturalist, XXXV., p. 91. 
