What did Wallace look like aged 31 – 39?Unfortunately there is only a single known photograph of Wallace taken between March 1854 and April 1862, when he was out in the ‘Malay Archipelago’ (right). He was photographed in Singapore in February 1862, shortly before his long voyage back to England, along with his close friend, the mining engineer Frederick Geach. Curiously Marchant’s 1916 book Alfred Russel Wallace; Letters and Reminiscences includes a ‘doctored’ version of this image, with Geach neatly painted out!

The only other photo of Wallace from this period is one taken in England by his brother-in-law Thomas Sims shortly after Wallace’s return from South-east Asia – probably also in 1862 (left). The only surviving contemporary copy of this image is a hand-coloured print in carte de visite format which belongs to the Wallace family.
See https://picasaweb.google.com/WallaceMemorialFund/ImagesOfAlfredRusselWallace#for (nearly!) all known images of Wallace.
Wallace's attire in the field
No detailed description of the clothes Wallace wore whilst doing fieldwork is present in his writings, but the outfit he was wearing in the famous photo of himself and Geach in Singapore is almost certainly not what he wore whilst out collecting. In a 1854 letter he mentions the "English preserve the tight fitting coat, waistcoat, and trousers, and the abominable hat and cravat" which is exactly what he is wearing in this photo. The reason he was dressed like this was because he was in a big city, not out hunting specimens.
Interestingly, Wallace is depicted in two woodcut illustrations in his book The Malay Archipelago. The woodcut showing him most clearly is “Ejecting an intruder” (right). His clothing is likely to be accurately portrayed in this picture, as it was based on a sketch he drew. Unfortunately, however, he is so small that the details of his outfit are difficult to make out (below). It looks like he is wearing an open necked shirt (minus cravat), a wide-brimmed hat and baggy trousers.
He notes in The Malay Archipelago that he wore waterproof leather boots.
In the text cited below he mentions that he wore a “hunting-shirt” whilst collecting. This is a shirt with at least two pockets – but it is unclear where these would have been positioned and also whether the shirt had buttons. It is probable that he didn’t have his shirt sleeves rolled up, as it is clear from
the following passage from The Malay Archipelago that the natives in Sarawak hadn’t seen the colour of the skin of his arms: “Many of the women and children had never seen a white man before, and were very sceptical as to my being the same colour all over, as my face. They begged me to show them my arms and body, and they were so kind and good-tempered that I felt bound to give them some satisfaction, so I turned up my trousers and let them see the colour of my leg, which they examined with great interest.” This is not
a surprise since sleeves would have protected his arms from the sun, thorns and insect bites.
In a letter from Sarawak in 1855 he mentions the collecting equipment he carried with him whilst out hunting insects:
"To give English entomologists some idea of the collecting here, I will give a sketch of one good day's work. Till breakfast I am occupied ticketing and noting the captures of the previous day, examining boxes for ants, putting out drying-boxes and setting the insects of any caught by lamp-light. About 10 o'clock I am ready to start. My equipment is, a rug-net [this is probably a misprint for “bag-net”1, which was what he called his net elsewhere], large collecting-box hung by a strap over my shoulder, a pair of pliers [wide, flat tweezers] for Hymenoptera [ants, bees & wasps], two bottles with spirits, one large and wide-mouthed for average Coleoptera [beetles], &c., the other very small for minute and active insects, which are often lost by attempting to drop them into a large mouthed bottle. These bottles are carried in pockets in my hunting-shirt, and are
attached by strings round my neck; the corks are each secured to the bottle by a short string."

a strap over the shoulder, and a little in front of the body on the left side, this will give the collector ample play with both arms and hands."
Endnotes
1. Salmon (2000) in his book The Aurelian Legacy: British Butterflies and Their Collectors says: "The earliest recorded date in the history of the butterfly net is 1711, when Petiver visited Holland and returned with the first known 'muscipula' or bag-net. This was a ring-net: a hoop attached to a stick on which a bag of muslin netting was suspended."