Make longtable captions single spaced using memoir class

Question

I am having a nightmare trying to solve this problem. It seems standard captions for figures and tables using memoir class are single spaced, but for tables created using longtable package they are double spaced. I can't make these single spaced using caption + setspace as memoir class doesn't load setspace, using its own code for baseline spacing and scaling (as I understand it). The most promising post I found was this one: Longtable Caption Spacing

I think it is suggesting to change the longtable.sty file in the latex distribution - adding \singlespacing??? If I'm correct, I have now done this but nothing is changing and the captions remain double spaced.

Any suggestions would be warmly welcome - been at this for a good couple of days.

Thanks.

EDIT: Here is a MWE

\documentclass[12pt,oneside]{memoir}

\usepackage{longtable}
\usepackage{lipsum}

\begin{document}


\OnehalfSpacing

\lipsum[1]

    \begin{table}[h]
        \centering
        \caption{Table very very very very very very very very very very very very very long caption}
        \begin{tabular}{llll}
            1 & 1 & 1 & 1\\
            1 & 1 & 1 & 1
        \end{tabular}
        \label{table}
    \end{table}

\setlength{\LTcapwidth}{\linewidth}
    \begin{longtable}{llll}
        \caption{Longtable very very very very very very very very very very very very very long caption} \\
        1 & 1 & 1 & 1\\
        1 & 1 & 1 & 1\\
        \label{longtable}
    \end{longtable}

\setlength\LTleft{0pt}
\setlength\LTright{0pt}
\setlength\tabcolsep{10pt}
    \begin{longtable}[htbp]{lcclcl}

        \caption[Specimen Collection]{Specimen information including species name, institution accession or record number, sex of specimen, host species name, year the specimen was collected, and location it was collected. Sex: F = Female, M = Male, j. = juvenile, t. = transitionary stage.}
        \label{table:museo} \\ \toprule

        \multicolumn{1}{c}{Species}  & \multicolumn{1}{c}{Acc./Record} & \multicolumn{1}{c}{Sex} & \multicolumn{1}{c}{Host}  & \multicolumn{1}{c}{Year Collected} & \multicolumn{1}{c}{Location} \\ \hline
    \end{longtable}


    \lipsum[2]

\end{document} 

You'll see in tables 2 and 3 the caption line spacing is 1.5, whereas, using a normal table, table 1 is single spaced. I've now changed back the longtable.sty file as that solution didn't work anyway.

EDIT: All Packages

\usepackage{lineno}

\usepackage{amsmath,amsthm,amssymb,amsfonts}

\newtheorem{thm}{Theorem}
\theoremstyle{definition}
\newtheorem{defn}[thm]{Definition} 

\usepackage{microtype}

\usepackage[dvipsnames]{xcolor}


\makeatletter
\newcommand\GIANT{\@setfontsize\Huge{20}{0}}
\makeatother  

\usepackage{titlesec}
\titleformat{\chapter}[display]
{\normalfont\bfseries}{}{0pt}{\GIANT}

\usepackage[numbered]{bookmark}

\usepackage{url} 
\PassOptionsToPackage{hyphens,spaces,obeyspaces}{url}\usepackage{hyperref}

\usepackage{textcomp}

\usepackage{color, colortbl} 

\usepackage[final]{pdfpages}

\usepackage{adjustbox}

\usepackage{multirow}

\usepackage{graphicx}

\usepackage{blindtext}

\usepackage{tgheros}
\renewcommand*\familydefault{\sfdefault}

\usepackage[T1]{fontenc}

\usepackage[helvet]{sfmath}

\usepackage[left=3.5cm,right=2cm, bottom=2.5cm, headsep=40pt]{geometry}

\usepackage{siunitx,booktabs,array,calc}

%\usepackage{setspace} doesn't work with memoir???

\usepackage{caption}

\usepackage{longtable}

\usepackage{etoolbox}

\usepackage{lscape}

\usepackage{rotating}

\usepackage{natbib}
\newcommand\cites[1]{\citeauthor{#1}'s\ (\citeyear{#1})} %adds possessive citation \cites{}
\newcommand{\citeay}[1]{\citeauthor{#1}, \citeyear{#1}} % comman in citealt
\bibpunct[, ]{(}{)}{;}{a}{,}{,}

Answer 1

Nice hack 1 (or ‘hack-patch 1’)

After some tinkering, I discover a (hopefully) better solution. Instead of manipulating \baselinestretch I suggest using the \fontsize{<fontsize>}{<leading>}-command. It is just to reset the the font size to the caption's fontsize (here 12 pt) and use the second parameter to create the single spacing. LaTeX normally set 20 per cent leading, so the correct command should be \fontsize{12}{13.2}\selectfont}. However, you may compress the caption by using 12 pt or 12.5 pt leading.

Using the \fontsize-command, you do not need to set the optional [Short caption].

In addition, I suggest defining a shorthand, \sspace so you can change leading one place. Maybe this even deserves to be called a patch, or at least a ‘hack-patch’ or ‘patch-hack’, since I use a font command:

\documentclass[12pt,oneside]{memoir}
\newcommand{\sspace}{\fontsize{12}{12}\selectfont} 

\usepackage{longtable}
%\usepackage{caption}
\usepackage{lipsum}

\begin{document}


\OnehalfSpacing

\lipsum[1]

    \begin{table}[h]
        \centering
        \caption{\sspace Table very very very very very very very very very very very very very long caption.}
        \begin{tabular}{llll}
            1 & 1 & 1 & 1\\
            1 & 1 & 1 & 1
        \end{tabular}
        \label{table}
    \end{table}

\setlength{\LTcapwidth}{\linewidth}
    \begin{longtable}{llll}
        \caption{\sspace Longtable very very very very very very very very very very very very very long caption.} \\
        1 & 1 & 1 & 1\\
        1 & 1 & 1 & 1\\
        \label{longtable}
    \end{longtable}

\setlength\LTleft{0pt}
\setlength\LTright{0pt}
\setlength\tabcolsep{10pt}
    \begin{longtable}[htbp]{lcclcl}

        \caption{\sspace Specimen information including species name, institution accession or record number, sex of specimen, host species name, year the specimen was collected, and location it was collected. Sex: F = Female, M = Male, j. = juvenile, t. = transitionary stage.}
        \label{table:museo} \\ \toprule

        \multicolumn{1}{c}{Species}  & \multicolumn{1}{c}{Acc./Record} & \multicolumn{1}{c}{Sex} & \multicolumn{1}{c}{Host}  & \multicolumn{1}{c}{Year Collected} & \multicolumn{1}{c}{Location} \\ \hline
    \end{longtable}


    \lipsum[2]

\end{document} 

Crude hack 1

Here is a crude hack that works, add \renewcommand\baselinestretch{1}\selectfontto the caption. For some reason, you must set the optional short captions [Long caption]:

\documentclass[12pt,oneside]{memoir}

\usepackage{longtable}
\usepackage{lipsum}

\begin{document}


\OnehalfSpacing

\lipsum[1]

    \begin{table}[h]
        \centering
        \caption{Table very very very very very very very very very very very very very long caption}
        \begin{tabular}{llll}
            1 & 1 & 1 & 1\\
            1 & 1 & 1 & 1
        \end{tabular}
        \label{table}
    \end{table}

\setlength{\LTcapwidth}{\linewidth}
    \begin{longtable}{llll}
        \caption[Long Caption]{\renewcommand\baselinestretch{1}\selectfont Longtable very very very very very very very very very very very very very long caption} \\
        1 & 1 & 1 & 1\\
        1 & 1 & 1 & 1\\
        \label{longtable}
    \end{longtable}

\setlength\LTleft{0pt}
\setlength\LTright{0pt}
\setlength\tabcolsep{10pt}
    \begin{longtable}[htbp]{lcclcl}

        \caption[Specimen Collection]{\renewcommand\baselinestretch{1}\selectfont Specimen information including species name, institution accession or record number, sex of specimen, host species name, year the specimen was collected, and location it was collected. Sex: F = Female, M = Male, j. = juvenile, t. = transitionary stage.}
        \label{table:museo} \\ \toprule

        \multicolumn{1}{c}{Species}  & \multicolumn{1}{c}{Acc./Record} & \multicolumn{1}{c}{Sex} & \multicolumn{1}{c}{Host}  & \multicolumn{1}{c}{Year Collected} & \multicolumn{1}{c}{Location} \\ \hline
    \end{longtable}


    \lipsum[2]

\end{document} 

Answer 2

The following patches \LT@makecaption at the correct point with the correct information for your MWE. Problem is that the macro is getting redefined and this removes the patch. If we patch \AtBeginDocument this should work, also we don't search for \sbox as the new definition doesn't use \sbox but we use \let as that is the first token inside the \parbox which surrounds the caption:

EDIT: I changed the answer a bit to so that it works whether the caption package is loaded or not. It could still fail if you use other packages which redefine \LT@makecaption.

\documentclass[12pt,oneside]{memoir}

\usepackage{longtable}
\usepackage{lipsum}
\usepackage{etoolbox}
%\usepackage{caption}

\makeatletter
\AtBeginDocument
  {%
    \@ifpackageloaded{caption}
      {%
        \patchcmd\LT@makecaption{\caption@@make}{\SingleSpacing\caption@@make}
      }
      {%
        \patchcmd\LT@makecaption{\let}{\SingleSpacing\let}
      }
      {}{\errmessage{patch failed!}}%
  }
\makeatother

\begin{document}

\OnehalfSpacing

\lipsum[1]

    \begin{table}[h]
        \centering
        \caption{Table very very very very very very very very very very very very very long caption}
        \begin{tabular}{llll}
            1 & 1 & 1 & 1\\
            1 & 1 & 1 & 1
        \end{tabular}
        \label{table}
    \end{table}

\setlength{\LTcapwidth}{\linewidth}
    \begin{longtable}{llll}
        \caption{Longtable very very very very very very very very very very very very very long caption} \\
        1 & 1 & 1 & 1\\
        1 & 1 & 1 & 1\\
        \label{longtable}
    \end{longtable}

\setlength\LTleft{0pt}
\setlength\LTright{0pt}
\setlength\tabcolsep{10pt}
    \begin{longtable}[htbp]{lcclcl}

        \caption[Specimen Collection]{Specimen information including species name, institution accession or record number, sex of specimen, host species name, year the specimen was collected, and location it was collected. Sex: F = Female, M = Male, j. = juvenile, t. = transitionary stage.}
        \label{table:museo} \\ \toprule

        \multicolumn{1}{c}{Species}  & \multicolumn{1}{c}{Acc./Record} & \multicolumn{1}{c}{Sex} & \multicolumn{1}{c}{Host}  & \multicolumn{1}{c}{Year Collected} & \multicolumn{1}{c}{Location} \\ \hline
    \end{longtable}


    \lipsum[2]

\end{document}